So what are you going to do for the other 7.5 hours in the working day? Its hardly fair to say that you will be doing that for the whole 37.5hours of your working week.
I charge Between £30 and £150 an hour for my laser cutters time, depends what they are doing, what quantities, what job etc. But they are not running all the time. This includes my unit rates, electric, rent, insurance, machine cost. websites, my time, my staffs time etc. Surely you as an accountant would know this?