Lisa,
Usually from sq ft, although some on here will/have slated me for it in the past, but I don’t think they do this type of clean anymore (wonder why?), measure the area/s in question, if its 50ft by 90ft that’s 4500 sq ft, then depending on the complexity of the clean ie: amount of fittings/windows/floor type/etc multiply by minimum 12p sq ft to max 25p sq ft.
A normal spec 3 bed, 2 bath, small kitchen, house approximately 1200 sq ft would be £144.00 (4 hrs max 2 people= 8hrs) for the initial clean, half for the second (£72.00) then £48.00 for the sparkle clean.
Always quote for the three cleans; if they only require one or two cleans adjust the sq ft price accordingly.
Always remember to weigh up the windows/fittings- accessibility/state etc, these are a big part of the clean and can take up the most time.
Regards
BSF
PS. Sorry I thought you were pulling my leg before, please accept my apologies.