There is no method really. To tell you the truth if you cant figure it out you probably should not be the one doing it. But you want to think about your crew or your self and how long will it take you to do it add up the wages and there is one part, then you think about supplies; i.e. paint and caulk and how much will you need to do the job. Then you need to figure out the cost of operation; i.e. Gas CHARGE, phone to do business.
Advertising CHARGE all these things need to be added together for one month and then divide by how many jobs you do on avg. Then your tools they depreciate so you need to CHARGE for that. Then lets us not forget the biggest, insurance!!! CHARGE and once you have all that then you have to out bid abc painting and or Joe blow who works for 5.00 an hour and don't care if he makes a good wage. So in short GOOD LUCK. lol Seriously though, be careful it just takes one job, miss bid, to ruin you and your company.