Break the windows down into a single standard unit.
A standard casement window has 3 panes of glass.
It has the narrow opening light at the top, usually a larger opening pane on the one side, and the fixed pane underneath the narrow pane.
The average time taken for such a window is 90 seconds
Charge whatever you see fit for a standard unit.
On georgian windows, if you are working trad, your standard sized unit, with 12/15 panes is going to take you 3 times as long to clean.
Personally I always allowed 6 georgian panes to equal one standard window.
Mostly the 6 georgian panes fit into the size of a single opening casement.
It also makes it easier to count up the windows.
On upstairs work, every move of the ladder must count as at least one window.
If, for arguments sake you decide to charge £1.00 per ground floor window, for a first floor window you should charge £1.25, 2nd floor, £1.75, 3rd floor £2.50...and so on.
Charge for leaded much as you would charge for georgian.
Have a minimum charge too!!
My own is £8.00...though I do 2 accounts, front only, 3 windows per account, ground, 1st and 2nd floor. and I only charge them a fiver each.
As they are next door to each other, the pair of them take not much more than 5 minutes.
So I guess you minimum can vary now and then!
But as a general rule, have a minimum charge. You will get people ask you for a price, you can at least say it won't be less than £££
Also of course you get to know your estates and the kind of houses they have on them, so you can at least give a ball park price when asked in the street or on the phone. Adjusting it accordingly when you actually get there.
But to recap, sort out what you think is a standard size window and have a price for that unit. Then break down the windows as you count them up.
15 windows? If you are going to charge 75p per window then its straight forward to simply multiply up, then either round it up or down to keep it to nice round figures.
Ian