Breaking down all of your expenses to a cost-per number is difficult. It took me some college courses to learn how to do it; that’s the best way to figure it all out.
Another option is to pay your accountant to help you figure out what you should be charging, that’s a great way to learn how to do it and to be sure you have good numbers to work with.
Probably the easiest way to figure things out is to work backwards, start with the profit you want to see at the end of the year, add up your projected expenses, your salary included. Divide the total by your estimated number of working hours for the year, and that will be an hourly rate to use.
Unless you’re working with multiple crews, or really specialty equipment, your cost per hour really does not change enough to count whether you’re using everything on a job or not. A skidloader sitting in the shop costs almost as much to own as the same machine working, so you need all your customers to help pay for it.
If you have multiple crews, things can get more complicated, you can bill one crew at a higher rate than another, but if you’re a one crew operation, your costs per hour will basically be the same for each employee, after the first crew member - that one has to cover the cost of the overhead (all your fixed costs, like the phone bill, website, insurance, truck payments…)
As I’ve said in many other threads, hire a good business coach, it will be well worth the money!