This is a question I ask people and the amount of different answers is great. It just goes to show you the art and science of determine your shop rate.
One metric I use to get my Shop rate is the full loaded cost of an employee. Taking into account what you pay an employee per hour is just a part of the equation. Factoring in benefits and vacation divided is important as well.
I think it would surprise people what the true cost of your employees really is. Once you do you can then start determining the profit you want to make based on a ton of other factors in the marketplace.