I tend to make it up as I go along(but I am only in my first month!) But I have devised a rough system I use which I put together from this site.In better off areas/houses I charge £1 a window,£1.50 for bay windows and patio doors,In poorer areas/houses I charge 75p per window and £1.00-£1.25 for bay windows and patio doors,If there are small windows I just add a rough figure on to the total price,normally to round the figure up to pounds rather than ponds and pence.I started with the idea of a £7 min charge,but I have since reduced that to £6,but that is for very small houses/flats.I have even gone done to £5 on a couple of properties,but they only had 3/4 windows and I could not bring myself to charge more,one said she was happy with that and would pay more if I thought it was not enough,and the other said the last cleaner charged £3,but I said that was probably why he dont come any more.So if the job is really small I will charge £5,but wont go any lower than that.