Name the user outcome
A button click is easy to count but may not represent success. Prefer a state that confirms the user achieved the intended result, then identify the observable signal that best represents that state.
Set boundaries
Write down who can enter, which route variants qualify and how long completion remains attributable to the start. Treat retries and repeated completions explicitly.
Keep a decision log
Definitions change as products change. Keep ownership, rationale and revision dates close to the metric so historical movement can be interpreted rather than guessed.