On some smaller jobs you might figure it by the day. Per opening works better if you have at least a few full days of work. The idea is:
If it only takes you 5-6 hours to rough do you have somewhere elce near by to make use of the 2 hours. Also the trip factor. Do you need to go back after drywall or sidding? More trips adds to the travel, set up, and clean up time.
Also if you estomate by the day you may give yourself more time if things don't go well. I allways figure more time then I think. I think it's an easy mistake to say "that wont take long".
Labor rate sounds low. You should try one of those online excel labor rate calculators. I don't have the address now. If your busy charge more.
Don't forget material mark up (your in business to make $ not a free shopper for customer electrical needs).