See Spendable Revenue
Budgeting off Sales hides pass-through costs and lets OPEX bloat. Use Real Revenue (Sales − Pass-through) and cap OPEX as a % of that.
Real Revenue & OPEX cap
Real Revenue = Sales − Pass-through. Set your OPEX cap as a % of Real Revenue, not Sales.
Optional: 14-Day Payback check for new spend
Payback days = spend ÷ (profit/sale × orders/day). Must be ≤ 14 for safe testing.