Why you work backwards
Growth teams that start from CPI negotiate against noise. Growth teams that start from allowable CAC negotiate against a defended number. The finance function owns ARPU, margin and payback horizon; growth owns install-to-payer. Multiplying the two gives allowable CPI. That number is the ceiling every media buyer, ad platform and channel test justifies itself against, or fails to.
ARPU and the payback horizon
ARPU = trailing 12-month revenue ÷ trailing 12-month active users. Payback horizon is a business decision, not a mathematical constant. Set it with finance according to the company's growth and profitability priorities. Publish both numbers, publish the review date, and stop re-arguing them mid-quarter.
Margin, not revenue
Every finance formula uses contribution margin, not top-line revenue. For subscription apps in India, store commission, payment failure, refund rate and customer-support cost per payer all subtract before the number is real. Use the app's settled finance data rather than a generic margin target.
Install-to-payer conversion
This is the single number growth owns. It rolls up onboarding completion, activation event completion, paywall view rate, and paywall conversion. Track it per cohort per market per OS. Do not assume India will match a US conversion benchmark for the same product. Diagnose activation, pricing and payment friction using local cohort data.
The allowable CPI derivation
Allowable CAC = ARPU × payback horizon × contribution margin. Allowable CPI = allowable CAC × install-to-payer conversion. Run a labelled scenario with the app's current ARPU, payback horizon and contribution margin to calculate allowable CAC, then apply the measured install-to-payer rate for allowable CPI. Spend above that ceiling needs a pre-registered incrementality test.
Reconciliation across MMP, Firebase and finance
MMP (AppsFlyer/Adjust/Singular) counts installs and post-install events with attribution. Firebase counts session-level product events. Finance counts settled revenue. The three will never agree exactly, but they must agree within a documented variance approved by finance and analytics. Weekly reconciliation is a discipline, not a chore.
When incrementality overrides allowable
Some spend is genuinely incremental, Apple Search Ads on your own brand term when a competitor is bidding on it, Meta retargeting of an abandoned trial, upper-funnel video that lifts organic. Incrementality tests (geo holdouts, GLL, PSA ghost ads) can justify spend above allowable CPI. The test must be pre-registered, powered, and published to the same audience that reads the CAC number. Ad-hoc 'we think it's incremental' does not qualify.
The refresh cadence
Allowable CPI refreshes quarterly on the same cadence as the ARPU review. Every refresh notes what changed (payback horizon, margin, install-to-payer per market) and re-derives. Every campaign flagged as 'above allowable' in the previous quarter is either killed, scaled with incrementality proof, or moved to a different objective. Nothing sits above allowable without a decision.