In order to stay in business you need to get enough to cover the rent of the building..how much would it cost you for a comparable building on the open market? Monthly utilities and phone bill. how much for consumables used in the shop? Advertising? cost of office equipment? Add all that expense up for the year and divide by 2000 and that gives a figure for projected expense of operations and do not forget the shop truck in that budget. Then figure your labor rate which is the expense of an employee (you) considering your hourly wage plus state labor and industries. social security taxes and unemployment insurance and then when you have all this together then a shop fee can be determined..
I have tried most all of it and now do what is known to work..