Start with contribution margin
Deduct product, fulfilment, payment and other variable costs from revenue before calculating the percentage left for advertising. Use revenue and costs on a consistent tax basis. At a 40% contribution margin, 2.50 in revenue contributes 1 towards advertising.
Break-even is a starting point
Fixed overheads, agency fees, refunds, timing and attribution can change the target your business actually needs. Compare ROAS with ROI, then explore paid advertising.