Start with the realised sale
Calculate the sale price from the displayed list price and discount. Then subtract the costs and transaction fees that still apply.
Look beyond the percentage off
A 20% discount is not automatically a 20% reduction in contribution. When the product already has a narrow contribution, the reduction can be much larger in relative terms.
Set a floor deliberately
Use a minimum acceptable contribution under clearly stated assumptions. That floor is a planning choice, not a guarantee that a promotion will be profitable.