Teams often arrive with two cohort definitions living in the same workbook: people who created an account, and people who completed a first purchase. Drawing both as if they were one population produces a matrix that looks precise and means almost nothing.
In kickoff we ask a blunt question: which event starts the clock for the decision you need to make this quarter? If the board cares about paid retention, first purchase wins. If the debate is about activation after signup, the account-created event wins. Mixing them for a single primary map usually waits until a later commission.
Once the entry event is named, window length becomes easier. Consumer apps with daily use often need weekly columns for the first eight weeks. Membership clubs with monthly billing usually prefer monthly columns for a year. The chart should match the rhythm of the business, not the default of the export tool.
Bring the definition to your next briefing written in one sentence. If two stakeholders cannot agree on that sentence, pause the drawing until they can.