I'm an electrician and I've done a lot of side jobs. I learned early on that after I had estimated my labor hours for a bid job I needed to double them for the bid. And when I was done with the job I found that doubled number was almost always spot on.
After about 20 years in the trade I got an offer to be a project manager/estimator. I spent 8 years doing that and learned a lot. When I first started, I was shocked to see the actual numbers contractors used to bid jobs. They were substantially higher than I expected them to be. There are so many factors I had not seen before when doing side jobs - vehicle expense, tool depreciation, estimating, design, sales and so many other things. I quickly learned those contractor numbers were necessary for survival.
Materials are easy to figure, it's the labor that will make or break you. Don't cheat yourself just to get the job. I agree with those who say $21/hr is low. Considering you are just outside of Atlanta I'd say that number is very low. The best way to know what the competition is charging is to get bids from them. Obviously you don't share that with the customer but it will give you an idea where you stand with local competition. Take those numbers and subtract your materials estimation and you can derive approximate numbers on labor rates, overhead and profit. Then insert those numbers into your formulas and see where you stand compared to your original estimation. And if they seem high, don't be afraid to use them, unless you are doing charity work.
“Travel is fatal to prejudice, bigotry, and narrow-mindedness..." - Mark Twain