Jack, You can set up a single field for each custom work project (one for each farm that you do custom work for). Use the total acreage that you did the custom work on. You can then record the farming actions for each custom project on these fields. Be sure to keep all of your equipment as passive enterprises. When you get paid for the custom work you should apply the income to the proper field (the field that corresponds to the farm that paid you).
When you do this, you will be able to print an enterprise statement for each custom field and see exactly how much money you made on each project. You will have total cost & profit as well as cost and profit per acre. This should help you to determine whether you are charging enough for your work.