Incremental thinking: The Growth CFO III

Every CFO knows the email. 

Subject line: “Diligence checklist”. Somewhere on page two, between the financial model and customer contracts: “Please send your fully diluted cap table.”

Six words that can ruin a week. Not because the math is hard, but because that cap table hasn’t been fully reconciled since the last option grant, or the last 409A. 

Pulley’s bet: that moment shouldn’t be stressful, because the cap table is already right. 409A, ASC 718, every grant, always on point. Email shows up, forward the file.

Could you send yours right now?

All finance professionals would be better for spending 12 months in a manufacturing costing role.

I know I was.

As a young pup, I didn’t quite understand it at first. I was obsessed with learning transferable skills, so I could move up the ladder quickly and fluidly. And in this costing role, I’d have to get into product-level detail very specific to the business: raw material grades, line speeds, packaging materials, etc. It didn’t sound very transferable…

But it didn’t take long to charm me. I quickly found the role much more cerebral and challenging than I expected.

The existing cost model leaned too heavily on cost allocations and theoretical absorption rates, none of which made much sense to me as a basis for decision-making.

So I rebuilt it, focusing on contribution margin, cost pools, and figuring out how and where those cost pools actually attached to individual products and activities.

It gave me a comprehensive boot camp into the discipline of incremental / marginal thinking and opportunity cost in the rawest, purest way possible.

And with it, I built one of my most valuable transferable skills…

In week one we defined the role of the CFO in growth, and how it's frequently over (or under) stated.

Last week we got deeper into what growth actually means, and how to capture what you're trying to grow and why;introducing the Value Atom.

This week we dive into the math of growth, and unit economics.

The ‘marginal thinking’ lessons I first learned in factory cost accounting years ago remain keystone for me. They are the lens through which I make almost every strategic financial decision; growth decisions most of all.

Let’s hop in…

The technical definition is the measurement of profit on a per-unit basis.

But I think about it more broadly than that.

Unit economics is the science of how your business makes money. And more importantly, what happens to that money as you scale.

The best CEOs and CFOs I've worked with all had an incredible feel for the unit economics of their business. They could quickly (mentally) calculate what a new store, product launch, discount, or new hire would mean for the business, in dollars and cents.

They were never exactly right. But they were right enough, in a fraction of the time it would take anyone else. From the first time I saw it, I knew that was the kind of CFO I wanted to be.

No putting the whole business on standby, drowning in weeks of analysis to make an everyday decision. They saved that for the genuinely close calls.

Think of it like Daniel Kahneman's System 1 thinking vs System 2. By making more everyday decisions from System 1, they freed up System 2 for the harder problems. The result is faster decisions and better execution.

But that speed isn't guesswork. It only works if you've truly internalize how costs behave in your business, how they attach, and how they move as you grow. That's what the rest of this post is about.

If you remember nothing else from this post, remember this: incrementality is the disciplined measurement of changes in revenue, cost, or cash caused by a specific decision.

Put simply, it's the precise difference between doing it and not doing it. Nothing else.

It matters because an income statement, by definition, shows you a position in total. Revenue is reported in total (broken into two to four segments, if you're public). Which means the margins (gross margin, G&A %, marketing efficiency) are all weighted averages.

And here's the problem: the average tells you nothing about what impact a decision might have. A decision only ever touches the increment, and the increment almost never looks like the average.

Income statements are reported on averages but that’s not how they are managed.

Great financial performance is the product of 1,000 great micro decisions at the margin, all pulling in one clear direction.

That's the hard part. Getting each marginal call right, day by day, while making sure they accumulate to the right answer in total.

So, how do we do that…

If I could take one metric to a desert island, it'd be Contribution Margin. The most under-rated number in finance, and the one I've carried with me into every room in every business I've ever walked into.

You won't find it on a public-facing income statement. It’s too commercially sensitive, too damn spicy for public consumption.

But it matters more than gross margin. And it's not even close.

Contribution margin = total revenue less total variable costs. It's the money left over to cover your fixed costs, and everything below them.

Here's why it beats gross margin for decisions. Gross profit is defined by GAAP, by how costs are classified. Contribution margin is defined by how costs actually behave.

It tells you precisely how your costs will scaleas you grow unit volumes.

Naturally, if you can build a chart of accounts that aligns cost behavior to classification, that makes this all much easier:

Let's work out an example. Imagine a single-product business, a DTC e-scooter company that manufactures in-house.

You could build a cost card and contribution margin statement that looks like this:

From here you can quickly get to a breakeven analysis, and start to build a pricing structure that incentivizes growth and maximizes profit.

But most businesses aren't simple, single-product examples. And most costs don’t show up wearing a ‘fixed’ or ‘variable’ name tag, the way they do in the textbooks…

The example above is a very two-dimensional version of the world. Reality is more nuanced: multiple products, different cost structures, different capacities for each.

So we need a framework that can handle contribution margin in a more complex world.

In real life, costs attach at different levels. Just as I learned in that factory years ago, some costs attach to:

It’s like peeling an onion.

The level a cost attaches at tells you what triggers it, and therefore whether a given decision will actually move it. Understanding how and where costs attach in your business is everything if you want to grow profitably.

Let's develop our example further…

Now imagine our little scooter business has two product categories, scooters and e-scooters, with two model variants under each. Each category has its own dedicated factory.

By building your P&L into a contribution margin grid:

For example:

Now we've really brought the P&L to life. We've got something we can take action on.

At the headline, the business looks healthy: a 10% operating income margin. But under the skin there's much more going on at the edges of the P&L, and those edges can have a huge impact on the total.

A few conclusions we could draw from this grid:

Our scooter examples assumes you pay to acquire each sale as you make it (i.e. it is not recurring business). But plenty of businesses pay upfront for customers and revenue that shows up later, particularly in any subscription-type model.

The trouble is how that world learned to measure itself. The SaaS explosion was built on managing unit economics as Lifetime Value (LTV) against CAC. There were two big problems with this:

The whole premise rested on a single bet: that if the LTV:CAC ratio was high enough, it would all work out in the end. Which it did… until it didn't.

And those habits have bled into lower-margin industries like e-comm, where it's frankly unforgivable given how COGS is so very real in a business like that.

Contribution margin analysis is rocket fuel for financial performance. But it comes with a health warning, something I call the Marginal Paradox. In the wrong hands, with the wrong controls, it makes it dangerously easy to justify profit-destructive decisions.

Let me explain.

Imagine a business with $10m of revenue at a 30% contribution margin. Take off $2m of fixed costs, and you've got $1m of operating income. A clean 10% margin.

Now an opportunity comes in: $5m of new business, but at lower prices. A 10% contribution margin, to be precise.

Marginal thinking says: that's an extra $500k of contribution, and fixed costs don't move. So we're now making $1.5m of operating income on $15m of revenue. The contribution margin percentage dilutes, sure, but so does the fixed cost percentage. Operating margin holds at 10%, on a bigger base. More dollars of profit in total. Yummy.

And here's the seductive part: it's right. That decision, on its own, is genuinely accretive. As I'm always banging on… you can't eat a percentage. Chase the dollars.

So where's the paradox?

That same "we're marginally better off" argument justifies any piece of business above 0% contribution margin. And very few businesses have perpetual revenue that never reprices. So the 10% logic starts leaking into the base. A discount here to retain an account. A price match there. Bit by bit, the 30% business gets renewed at 10%.

Take it to the limit: all $15m eventually repricing to a 10% contribution margin. Now you've got $1.5m of contribution against $2m of fixed costs. You're losing money, on a bigger revenue line than you started with.

In a small business this is easy to control. Pricing discipline sits with one or two people. But in a large business, with hundreds of people each making "marginally better off" micro-decisions, every one of them defensible in isolation, the base can erodes one reasonable-sounding call at a time.

No single piece of volume has to cover the fixed costs and the profit on its own. But enough of them do, in total.

The art of great growth management is making marginal decisions correctly, one at a time, while never losing sight of what they have to add up to.

Now, I've talked a lot about cost behavior. There's just one problem… costs don't always behave themselves.

Let's go back to the single-product example from earlier. A classic breakeven chart would look like this:

Clean, isn't it? Two straight lines, a tidy crossover, fixed costs sitting flat as a table. The trouble is that's not how costs actually behave. In truth, nearly all costs are mixed: part fixed, part variable, with a tendency to lurch as you scale.

So you need to button down the relevant range for a particular cost structure. A range in which it's reasonable to assume individual costs hold as fixed or variable. You can bring this to life with a Volume Step Cost analysis, showing how the cost profile develops across different volume steps (in this case, 50 units at a time):

Note the units on the horizontal axis are units sold, not time. The time period is fixed.

The assumptions that sit underneath this example:

And how do you calculate a Volume Step Cost analysis? For each cost step, ask yourself: how would I operate at this level of volume in the cheapest way possible? If you don't know, find out. Speak to suppliers, examine your capacities, do the math. It’s really important, and should be more than educated guesswork.

Now our 'theoretical' breakeven chart looks very different given a dose of real life:

And look at what it does to the bottom line. Operating income doesn't climb smoothly with volume. It lurches. Every time a fixed cost steps in, profit takes a hit before the new volume fills the capacity you just paid for.

So knowing how and when to make those fixed capacity investments becomes critical. This is what I meant back in Part I when I said the CFO's role in growth is to know how and when to throw the sandbags off, not fire the burners.

So how do you make those decisions?

Now let's go back to a simple example, but imagine it at a much larger scale.

You're planning to add new warehouses to cope with an explosion in demand. A second, a third, a fourth, and so on.

This shows up as a step up in fixed costs at each capacity point. Each step is a new warehouse, opened to carry the extra volume.

Here we've illustrated the impact of a 2nd, 3rd and 4th warehouse. You can see each new cost 'stepping in' at its capacity point. Now let's imagine demand grows so fast you commit to all four.

Then disaster. A major demand shock, and your volume halves. You've got the fixed cost base to merit the higher volume, but not the volume. And it's not as simple as switching it off again. You're locked into rent agreements and other fixed overheads.

Now you're carrying stranded cost: any fixed cost in a business that isn't rational for its current volume.

The efficient cost curve is the blue dashed line. The red shows the actual cost curve including the stranded cost. The gap between them is the in-built inefficiency of running sub-scale.

The same applies to fixed cost invested ahead of growth that never shows up. A cardinal sin (and embarrassing for a CFO.)

So getting the call right on how and when to step in a new fixed cost is one of the highest-stakes judgments you make. Too late and you choke growth. Too early and you bleed inefficiency.

The same logic applies to growth-related CapEx and working capital. Your commitment to this kind of sticky, semi-permanent investment has to match your level of conviction.

You can also use the power of the checkbook to force the issue. When I found myself trapped between capacity points, inefficiency and stranded cost on one side, and missed opportunity on the other, I gave my sales team a very clear message: Here's the new business commitment I need to see to unlock the next round of investment. Often there was a cornerstone customer, or one big commitment, that would underwrite most of the cost for a period. Sales people are great with a very specific target, especially if delivering it is in their interest.

Staple the big investment moments to a big commitment. It forces the whole business to think properly about the economics and sequencing of scale.

Oftentimes, these kinds of decisions boil down to opportunity cost.

It’s not just ‘yes’ or ‘no’, it’s understanding that when you explicitly say ’yes’ to something you are often implicitly saying ‘no’ to something else.

That’s a good thing, as it forces focus and a concentration of resources. But as CFO, it falls on you to vocalize these trade-offs  and make them explicit.

They may be obvious to you, because you have a 360 view of the constraints on the business, but to your sales, operations, or product teams? Not so much…

They may not see that ‘yes’ to this is also ‘no’ to that. The more you help them understand that explicitly, the better decisions the business will make.

Big growth moments means (for the CFO) big decisions on when to take constraints off. But constraints are there for a reason; they are your cash and spending safety belt. The faster you are growing the more frequently you will encounter these moments.

The key to getting them right is being clear on how much conviction you and the business needs in order to make the step. And then comparing that conviction to the volume and quality of demand commitment you have.

But it can’t just all be about your mercurial CFO instincts. You have to institutionalize these skills into your finance function… and we’ll focus on that next week.

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Disclaimer: I am not your accountant, tax advisor, lawyer, CFO, director, or friend. Well, maybe I’m your friend, but I am not any of those other things. Everything I publish represents my opinions only, not advice. Running the finances for a company is serious business, and you should take the proper advice you need.