Trying to get a new office cleaning business off the ground

Trying to get a brand new office cleaning business off the ground.I have researched the number of companies in my area and tried to get a feel for what they charge.The biggest problem that I have is that I don't know what formula to use when I would be ask to visit a customers office to give an estimate.If anyone has any information that they think would be helpful it will be very much appreciated.Thanks in Advance!