How do you calculate what to charge for pressure cleaning walkways and driveways?

Ok heres a bit of information , you should charge by the foot of walk ways , and make it so you come out a little better per hour then if you were working for a wage plus , use of your washer and milage, then take your working wages , and duduct 10 % for breakdowns, if a drive way is figured, on square footage then , figure it the same way as a side walk a side walk could be 36 inchs , wide , and 50 foot long , now figure how much time it costs you to do this with man hours, you should get 25.00 per hour for your labour, + charge what ever you think it costs to use your washer ,. i dont know the size or the power , or what kind of set up you have, but take 10% for breakdowns and put it away , plus cost of gas or whatever fuel you use, could be another 7 % so 17 % needs to be put away, good luck!