From my perspective -- working on a watch repair branch -- I try and get a fair price per job but I make no attempt to make consistent money per hour for the branch, if for no other reason than it's futile to try.
Sometimes you spend a couple of quiet hours doing one complicated thing for the same amount of money you spent 20 minutes earning when the customers are rolling in with loads of simple work. You kind of concern yourself more with your revenue per week than stressing out about the micro of minutes and hours.
If i worked in a more private controlled setting, i don't know how much my attitude would change to that, Id still make how long a job was likely to take me a fundamental part of the equation as to how much id charge, but I'd still end up with some jobs being far more or less profitable than others, im sure. For one, you just can't ever reliably predict how much trouble something's going to give you.