Steve
It is really important to price your work correctly.
I think that generally speaking there are too many variables to be able to price a property by mealy guessing or counting window.
So, when somebody asks me for a quote, I tell the individual that the most accurate way to price their property would be for me to time the second clean. I explain that the first clean will take a lot longer to bringing the frames, glass and sills up to a standard that I can maintain thereafter.
I also explain that although I will lose out a little on the first clean I am nonetheless prepared to undertake the initial clean for such and such a price and then give them the more accurate pricing on completion of the second clean, whereupon they can at that time decide on whether the price is agreeable.
You may only make half your hourly rate on that first clean but the customers really seem to appreciate the extra work, and invariably it will leave them with a good impression of you.
Nick