I have been detailing for over 12 years. I have found that charging by the hour is definitely the sort of thing you dont want to do. especially on scratch repair. You can go in and polish out a scratch in 5mins and that wouldnt make you lots of money. My point is always charge based on the job itself, your skills/worth, and the end result.
example: 6 inch deep scratch on door. Where im from it will cost anywhere from $450-$600 to paint the door(keep in mind they get what they pay for) I know i have to prep,fill in, touch up, wetsand, polish, sealant/wax(both provides more value) i also know this is a slow process if done correctly.(more time=more money), than my skill(how confident are you in your work) for a job like this i charge anywhere from $130-$200. an no i dont feel bad about charging $200. i just saved them atleast $250, kept as much of the factory paint untouched, and did one hell of a job. with that in mind i know of shops that will charge $80. as stated before, they get what they pay for. Do you want penny pinchers or do you want customers who appreciate your skill and will stay loyal to you because you take the time to do the job right and are happy to pay you for it..