Take for example, I charge 1.75 sq foot for 10x10 room with 8 foot ceilings. By my calculations this would cost $175 with floor space sq foot, but with you method, it would cost $576.00 That is outrageous. Of course I didn't subtract the windows and doors, which is another thing that makes no sense. Cutting in around those windows takes more time, then just rolling a wall.
Basic labor to paint home with favorable site conditions. Prep up to 3 hr per 100 SF - clean, scrape, sand and patch up to 4 defects per 100 SF. Caulk gaps and spot prime. Roll / brush 2 coats of paint over lightly textured ceiling/walls. Paint up to 2 doors and door trim per room. Includes planning, equipment and material acquisition, area preparation and protection, setup and cleanup. 129 square feet $97.99 $132.56