What is a good profit margin for a repair shop?

There is no single correct number, and any tool that gives you one without knowing your market is guessing. What matters is the shape: under 10% net margin after overhead leaves nothing for a warranty return, a dead part, or a slow month, so a shop running there is fragile even when it looks busy. Between 10% and 25% is workable. Above 25% net you have room to absorb a bad month. Judge your margin against your own break-even and your own costs, not against a figure from someone else’s market.