If you know how long it will take then you can work an estimate out. The difficulty with repairs is that you don't usually know how long it will take as you often do not know the issue and by the time you have diagnosed it, you can be well into the time it will take to finish it.
Sometimes I have diagnosed a fault but need a part so I can estimate the cost of continuing. I would work it out at our rates x the time I think it might take, add 25% gross onto parts as a minimum and be clear that it is an estimate and will not cover other issues that could arise, particularly on older appliances.
Hope that helps.
If you are feeling guilty about over pricing something, don't. It could have gone the other way. That is the reason we don't tend to give prices for repairs, it is difficult without knowing what needs doing.