Profit margins are the amount above basic operating costs to build revenue. Figuring your profit margin is accomplished by using a percentage, which is is based upon the sum of the previous two items above. To find the hourly cost to run your business profitably you need to take your hourly pay plus your overhead, then divide that by a percentage to find a number for your profit margin.
I'll use a contractor job as an example. If I wanted to earn $15 per hour, and a job took me an hour to finish, I'd make $15. The supplies I used to do the job would include materials, gas, permits, insurance, and so on. Let's use $50 to cover overhead for the job. It looks like this: $15 + $50 = $65. If I want a 20% profit I would take 20% of 65 which is 13 – this would give a profit of $13 per hour.