How many hours a year do you have avaliable to sell ?? Your own ?? Hired hours ?? Minimum charge of one hour for anything you do on any site. Hourly cost is overhead, labor and any other costs, ( not materials) divided by how ever many hours you are going to sell. The one problem with the way you are doing it now, is that there are hours associated with on site production that you need to recover from each account. Very seldom can you do that by just charging an amount per hour and hoping you cover those times. Those hours would be drive time, shop and repair times, dump times, supply procurement and other mobilization times.