Every forecast carries assumptions: how many deals will slip, what the win rate will be, how much expansion lands. Scenario planning writes them down.
Best, base and worst case should all start from the same pipeline. Only the assumptions change, so the gap between scenarios is easy to explain.
Move win rate, slippage and expansion separately. You learn which assumption moves the number most, and where to focus the team.
Give finance all three numbers and the assumptions behind them. Agree which one the plan uses before the quarter starts.