KG,
You might consider doing a few jobs on a
Time and Material basis. before you jump into sq ft bidding.
Just quote a hourly rate with a estimated completion time. You can let the customer provide materials or if you provide them add 25%- 30% to the retail material cost for your handling / setup.
A lot of big contractors are using T&M bids as opposed to sq ft. simply because you can't get burned with add ons, hold ups, and unexpected time delays.
Good Luck,
rmichael pro painter