Sean is on the right wave length here. Rather than pricing per square foot, you may want to consider knowing your cost per hour of operation.
Too many contractors have no idea why they charge what they charge. Even if you want to use a square footage methodology, you must know what the cost per hour is as a basis and then break that number down to a productioncapability per hour based on equipment being used.
95% of the contractors have no idea why they charge what they charge other than they are pricing based on their competition. This type of pricing is ok if you have a base rate per hour and you knopw why. Otherwise, you may spend a lot of your time thinking you need more work instaed of charging enough for the work you already have.
Just a few thoughts!
Good luck and have a profitable year