Markup is not margin
This confusion costs contractors real money. Markup is added to cost; margin is taken out of price. A 50% markup gives you a 33% margin, not 50% — and if you priced a year of work believing otherwise, you were a third short on every job.
Result
Margin = profit ÷ price. Markup = profit ÷ cost. To hit a target margin, divide your cost by (1 − margin) — not by adding the margin as a markup.
Questions
Is a 50% markup a 50% margin?
No. A 50% markup on $100 of cost gives a $150 price and $50 of profit, which is a 33% margin. To get a 50% margin you need a 100% markup.
What margin should a painting company target?
That depends on your overhead, which is what the hourly rate calculator is for. The important thing is knowing which of the two numbers you are actually setting.
Or stop doing this in a calculator
Aerys prices the whole job from your own catalog and production rates, sends it with an accept link and an invoice already attached, and then tracks what the job actually cost against what you quoted.