Opening the book…
When one metric controls someone's livelihood, the cost of missing it swamps the cost of distorting it, and any rational person optimises for the number rather than the thing it stands for. A single number is also a single attack surface: there is one way to win, so everyone finds it. Spreading consequences across several measures plus a judgement call makes gaming more expensive than doing the work.
If a number feeds compensation or promotion, pair it with at least two others that pull in different directions, and reserve part of the decision for a manager's written assessment. Keep the highest-stakes decisions — firing, bonuses above a token amount — anchored in review of actual work rather than a threshold crossing. Say out loud which numbers are informational and which are consequential, and keep that list short.
Straight commission on verified revenue, or piece rates where the unit is physically inspected, can run off one number because the number and the work are the same object. Very small teams where a manager sees all the underlying work directly need less scaffolding.