Most founders ask "how do I get more customers?" before the only question that matters: do I have a product people would miss if it vanished? Sean Ellis turned that into a single test — and Superhuman used it to climb from 22% to 58% before spending a dollar on growth. Here's the mechanism, and how to run it on your last 30 customers this week.
The mechanism: measure the miss, not the like
When you ask a customer "do you like the product?" they hear a judgment of you — so they're polite, and the answer is worthless. Ask instead "how would you feel if you could no longer use this?" and they picture the loss. Reactions to loss are honest, because people feel the pain of losing something more sharply than the pleasure of gaining it.
Sean Ellis studied around 100 startups and found one line that predicted which ones would grow: the share of users who'd be "very disappointed" to lose the product.
The 40% rule
Ellis set a threshold. If fewer than 40% of your users say they'd be "very disappointed" without your product, it isn't ready to scale — and marketing will only spread a weak product faster.
The trap is the middle answer. "Somewhat disappointed" feels like progress, but it isn't your signal. Only the "very disappointed" group tells you what makes the product indispensable.
The proof: Superhuman
Sources: First Round Review and Mind the Product on Superhuman's product-market-fit engine.
Rahul Vohra's first measurement came back at 22% — barely half the bar. Most founders would have kept spending on growth and hoped. He did the opposite: he stopped, studied exactly who said "very disappointed" and why, and rebuilt the product around them. The score climbed to 58% in months. Only then did Superhuman scale.
They didn't spend on growth while weak. They spent once they were strong.
Why this matters before any ad
Marketing amplifies what's already there. If the product is strong and people are attached, marketing carries that strength to more people, faster. If the product is weak, marketing carries the weakness to more people, faster — burning your budget and your reputation at the same time.
The real question before any campaign isn't "is my ad good enough?" It's "is my product good enough to spend on?"
Run the test — the 3 steps
1Ask your last 30 buyers
Real buyers, not leads or people who were "interested." Their answers are the ones that count.
2Use the exact wording
"How would you feel if you could no longer use [product]? (a) not disappointed (b) somewhat disappointed (c) very disappointed."
3Read the number
What to do with the answer
Ignore the "somewhat disappointed" group — chasing them waters the product down. Study the "very disappointed" group instead: what do they have in common, what do they use it for, what would break if you removed it? That is the core you rebuild and scale around.
Before you ask "how do I get more customers," ask "would they miss me?" One question, thirty customers, and a number that tells you the truth: scale, or fix first.
Accuracy note: the 40% rule is Sean Ellis's, not Superhuman's — Superhuman applied it and built an engine around it. The 22% → 58% figures are from its documented case study.