Start from the business, not the platform
What revenue do you need, at what margin, and therefore what can you afford to spend acquiring it? That is the ceiling.
Shape it to your curve
Look at last year’s revenue by month. Budget should lead the curve by two to three weeks, not follow it.
Leave room
Hold back a reserve for the moments a campaign is capped and profitable. That is the highest-return money in the year and a rigid budget prevents you spending it.
Revisit quarterly
Costs change, margins change, the plan built in January is wrong by June. Recalculate break-even rather than defending the spreadsheet.