How do I know I’m charging enough but not to much

When I look at our business  I know: 
My fixed monthly operating expenses, our expected tax liability, and how much money needs to be made in a year to live a comfortable life. 
So to bid jobs we do labor hours at a fixed rate + materials costs (with no real mark up... awful I know, but we do have a general supplies type of fee. I may even do a markup percentage on the materials to cover the fixed monthly operating expenses.....) 

Lets say I don't know how long a job will take but I know it is a small to medium job that could be done in either 2hrs or take 4-6hrs depending on how everything shakes out like a window replacement in brick Well then I default to bringing in $400 profit a day on a bad day for a job that goes longer still pays all the bills. Like if you did that everyday it would be 96k a year before tax working 5 days a week, 48 weeks of the year. Which is fine (till you consider taxes haha so don't have too many bad days). But if you account for as many of the materials that you will need to buy or use in the job itself and place them on the bid... then you are only out labor if something goes wrong. Which is not fun... but it is better than being underwater in both. 

So that is how I also gauge it out, then if the work is commercial I bill at a slightly higher rate than for residential (since commercial comes with more requirements from us as a business).