Well, you need to know your costs...which are fixed, and which aren't.....
As an example, insurance is fixed, so much per year. Divide that into your expected number of working days/hours per year, and that will give you a rough cost per hour of machine time.
Fuel, well, you should have an idea of how much fuel it uses, whether by day or whatever, so you can figure it out by the hour from that.
As to what I charge....that should be determined by your costs, plus a markup....then compare that to others in your area to make sure you're not at one extreme or the other.....
Since I only have the one mini ex right now, I also compare to the price to rent a similar machine...I try to have my 3 hour minimum about the same or a little less than a rental... most will realize that I can do more in 3-4 hours than a complete novice on a rental can in a day, and in tight spots there's no comparison to experience.
Reality is, you need to know your costs, to determine what you need to charge.....and hopefully it is less than what you would like to, and can charge.....that difference is the extra profit to be made.