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 is trailing twelve-month revenue divided by trailing twelve-month active users. Agree the payback horizon with finance in line with growth and profitability priorities. Document both figures, their sources and the next review date.
Margin, not revenue
Every finance formula uses contribution margin, not top-line revenue. For recurring-revenue 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
The MMP counts attributed installs and post-install events. Firebase records product events, while finance records settled revenue. Reconcile the systems weekly and document the acceptable variance with finance and analytics.
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
Refresh allowable CPI quarterly alongside the ARPU review. Document changes to the payback horizon, margin and install-to-payer rate by market. For campaigns above the threshold, agree whether to stop, change the objective or continue based on incrementality evidence.