A fair percentage is whatever you need to cover your expenses. You have time invested to pick up the materials, drive back to the job and provide a warranty as well as waiting for your money until you get paid.
If you think things will go bad by charging too much, consider what will happen to your business if you charge too little.
I had an auto repair shop that worried he was charged too much, and his way of thinking, almost drove everyone out of business, he was not able to do jobs correct, never charged eniough, truck would break down when we needed it to make money, his thinking that everyone wanted cheap, was the worst business give he made, he was not being fair to his customers, by offering cheap. People in business understand the real cost of operating a business.
