I bought some window washing equipment and will be ready for some work this spring. How much should I charge?

Hourly? Square foot? And height? I have equipment to go up to 36 feet high.I bought some window washing equipment and will be ready for some work this spring. How much should I charge?
My brother and nephew own a window cleaning service and have the contracts for major office buildings, hotels and an Indian casino. For normal home windows, they charge $4/window. For commercial windows, it's $5 per window (inside only) and $8/window from the outside. If they go to the roof and come down, it's $10/window. When they tried charging by the foot, people were haggling too much. When it's a flat fee, it's the same as they were going to charge by the foot, but no one haggles because they can't argue about measurements.