Paul there are alot of factors to consider.
We clean all frames, top edge and vents, all doors including garage doors. We use around 500 per 1 man van per day.
If we where just cleaning glass we could half the amount we use.
I know for a fact we over clean, BUT, we never have any call backs, never lose customers for quality reasons and are expanding rapidly so I am not bothered. Hope this helps with your calculations.