Contribution margin is the number every founder should memorise
Contribution margin (price − variable cost) is what each sale contributes toward fixed costs and profit. Under 30% → hard to scale.
Software has 90%+ contribution margin — that's why SaaS valuations are so high. Restaurants sit at 25–35% — every seat has to work harder.
Break-even shifts fast: a 10% price cut can double the units needed to break even. Test pricing before you cut it 'to move volume'.