Estimating by square foot is a risky way to cost a project. get too high you will not get the job, too low it will cost you. Square foot cost should at best is a ballpark estimate, not a quoting tool.
I typically do my estimating per device and type of device. What I do and maintain is a work sheet to do assemblies with. For example and the costs do not reflect any actual costs and material:
Duplex receptacle $1.00
Faceplate $0.89
One gang nail on box $2.89
20 feet of 12/2 Romex $5.00
Six Romex staples $2.37
Labor for boring and installation $25.00
Assembly total: $37.15
I make an assembly cost for each item then when I do a take off, I just count up each assembly, multiply the cost, then and OH&P the viola!, a good estimate. More assemblies you have, the more accurate the cost would be but more work. If you have a spreadsheet program, you can build your own templates and by dumping the unit cost of you materials at the top, the quantities in the middle, at the bottom you should have a good, solid estimate every time.
