Margin of Growth: Master The Math That Powers Sustainable E-Commerce Growth

Your Shopify dashboard says you’re winning. Your bank account says otherwise.

This is the paradox of modern e-commerce.

You hit revenue milestones.
You scale ad spend.
Sales notifications roll in all day.

Yet when it’s time to pay vendors, place inventory orders, or take a distribution, the cash isn’t there.

That gap isn’t bad luck.
It’s math.

Most founders are running their businesses on vanity metrics—optimizing for top-line revenue while ignoring contribution margin, cash flow, and payback timing. They confuse growth with health.

The solution isn’t choosing between creativity and discipline. It’s learning how to simultaneously think like a Marketer and operate like a CFO.

Margin of Growth is a practical framework for doing exactly that.

Who this book is for

This book is for:

• Founders tired of guessing
• Operators who want financial clarity, not hustle culture
• Business owners who want to build an asset—not a stressful job

If you make decisions about marketing spend, inventory, pricing, or growth pace, this book is for you.

What you’ll learn

This is not a branding book or a source of creative inspiration.
It’s a technical playbook grounded in finance and operations.

Inside, you’ll learn how to:

• See your true unit economics
  Use the Unit Economics Waterfall to track where every dollar actually goes.
• Run your business on contribution margin
  Why gross margin is misleading—and how to manage the number that really matters.
• Avoid the inventory cash trap
  Identify hidden working-capital deficits before they erase your runway.
• Calculate your allowable CAC
  Spend aggressively on acquisition without putting the business at risk.
• Forecast instead of hallucinate
  Replace optimism with models that show where your cash position is heading.

About the author

Tom Santagato is the founder of FourThree Media, an e-commerce growth agency and consultancy.

He competed internationally as a member of the U.S. Skeleton Team while completing his MBA in Finance, then spent several years working on Wall Street before founding FourThree Media in 2017.

Over the past eight-plus years, he has worked with e-commerce businesses ranging from early-stage founders to established brands managing multi-million-dollar budgets.

The promise

Most founders don’t fail because they lack intelligence or ambition.
They fail because they ignore the math when the story feels more exciting.

This book gives you the frameworks to do the opposite.

Stop guessing.
Trust the math.
Build a business that survives the scale you’re chasing.

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