If you're hiring an engineer, then I would think that a plan of attack might be to first figure how much this guy is costing you. Is he billing you by the hour for his services or are you paying a salary?
Either way, you probably should come up with an hourly figure for how much this guy is going to cost you. If he's billing you hourly, then you'll already know his hourly cost. Then come up with a reasonable target figure for the number of billable hours you'll be able to generate per month; i.e.how much business you plan on doing. If you paying him a salary, take his monthly pay and divide it by the number of hours you be billing for, and that will tell you how much per hour this guy is costing you.
Then figure out how much everything else is going to cost you per month (rent, equipment loans, insurance, advertising, expendables, other employees, etc.) Divide that by the number of billable hours you plan for. Take that answer and add it to the engineer's cost per hour and you'll have a rough cost per hour for you to do business.
Add what you feel is a fair percentage for you to make for yourself and you'll have a rough idea of what your hourly rate needs to be. If it's too high compared to the competition, then you'll need to either find a way to increase your billables or decrease your costs. If it's too low compared to the competition, then either your estimate of billable hours is unrealistic for your market, or you have managed to keep the costs low enough to have a winning business formula.
(And to get you off to a good start, I won't bill you for this consultancy
)
G.