Al little of both. For larger lots you go by how much time we figure it will take, or charge by the hour. Smaller drives I have a minimum so that I don't get loaded up on them. Hate to harp on this but there is a lot of good discussion on this previously, just try the search. Welcome aboard.
Second thing is to know how much it costs you to do business per hour (with indirect costs, overhead, ins. etc.) and then what you need in profit as well as your direct labor expenses. These are key numbers as wel as how long it takes to complete the project and leaves yourself some time on the plus side as a cushion.