Marketers compute LTV by dividing one by the churn rate. Actuaries have spent a century learning why that number is a work of fiction.

1. One divided by churn is not a lifetime. It is a lifetime only if churn is constant forever, which it never is. Early customers leave fast, survivors leave slowly, and the blended monthly rate you are dividing into one is an average of two completely different populations behaving in opposite directions.

2. Build a triangle instead. Every cohort gets its own row and ages across the columns – month one, month two, month twelve. This is the chain ladder, the oldest tool in casualty reserving, and it exists precisely because the number you post early is never the number you end up paying.

3. The tail is where the money is. Casualty actuaries live in fear of adverse development – losses that keep growing years after the policy expired. Your retention curve has a tail too, and it is quietly deciding whether your acquisition spend was rational.

4. Cohorts confess. Averages lie. A flat blended retention number can hide a business where every recent cohort is worse than the one before it. Triangles make that visible in about ten minutes, which is roughly ten months before the blended number does.

5. The arithmetic of keeping people is not close. Reichheld and Sasser’s work at Bain put a 5% improvement in retention at somewhere between 25% and 95% more profit, depending on the industry. Nothing on the acquisition side has ever produced a range like that.

6. You need less data than you fear. Twenty-four months of billing exports and a spreadsheet is enough. This is not a data science project. It is arithmetic with the rows arranged properly, and most companies have never done it because nobody in the building was taught to arrange them that way.

7. Pick the point where the curve flattens. Every cohort chart has a month where departures stop and the survivors become an annuity. That month is your real payback horizon, and it is the only defensible input into what you are allowed to spend to acquire someone.

8. Then re-forecast, out loud, on a schedule. Reserves get reviewed quarterly and restated without embarrassment, because everybody understands the first estimate was an estimate. Marketing forecasts get defended to the death. Adopt the actuarial manners along with the method.

9. Show the client their own triangle. Nothing in a strategy engagement lands harder than a founder seeing their own cohorts laid out for the first time. It is their data, it takes an afternoon, and it usually ends the debate about the marketing budget without anyone having to win an argument.

The bottom line. An insurance company that guessed at its liabilities the way most software companies guess at lifetime value would be in receivership by the third year. The tools to do it properly are a hundred years old, free, and sitting in a discipline nobody in marketing has bothered to read.