Your product's gross margin before the discount (selling price minus cost, as % of price).
The discount offered to the customer, as % of selling price.
Volume lift needed to break even
Minimum incrementality required
Margin per unit after discount

The math: break-even volume lift = margin ÷ (margin − discount) − 1. Minimum incrementality = discount ÷ margin (the share of discounted orders that must be genuinely new for the incremental margin to cover the margin given away on orders that would have happened anyway). Both formulas and how to measure incrementality with holdouts are covered in The Promo & Discount Playbook. This is gross-margin math only — operational promo costs make the real bar higher.