I charge everything by a square ft formula. This is much more profitable than a time and materials deal. It gives you incentive to be efficient. The customer has no need to know how much you make per hour, and I do not want them to know. Find out what the going rate per 1000 sq ft is in your area. Here is a suggest pricing schedule
0-8m sq ft = base price or minimum.
8-20m = x per 1000 sq
20-40 = 10 to 20% less than 8-20
40 up = 10-20% less than 20-40