Don't charge by the square foot period. The jobs you don't get using that method you probably where over priced. The jobs you do get you where probably the lowest price undercutting yourself.
Price your material, labor, overhead, all marked up and add your profit.
Ask yourself this theoretical question. Are you going to price a wall that is 100 feet long and 1 foot high the same as a wall that is 10 feet long and 10 feet high? Both are 100 square face feet.