At the end I total up the sections add PAT and write up quote.
Paul who the hell is PAT ?? 
Haha wondered who would be first to ask.
It's "Paul Added Tax".
A bit extra added to bill to cover anything I missed when quoting or a layer to protect any haggling they might do.

Agree.
Schools usually only want the windows cleaned once a year. Some of those bird poo strikes can be a nightmare to completely remove as they have been baked on the glass.
If you think it will take you 20 minutes, wrong; it will take 30 minutes.
We started off with a new school building 8 years ago and each year its getting harder to achieve the time schedule I originally set. I'm also 8 years older so this could also have a bearing on the results.
I have the school buildings drawn up on a spreadsheet. I've broken the school down into sections and we time each section as we clean it. We then compare the time taken for each section over the years.
The other issue you need to consider (it was in our case) is that they generally want the school cleaned during the last two weeks of the summer holidays. We are inconvenienced by exams results becoming available on 2 of those days. They don't want us on site on those 2 days. So the questions are always; when do we start, when do we need to be finished and when are the exam results days. We then schedule our time around that taking the weather into consideration.
It helps as I have keys to the main gate and all the other internal gates, so we can work on a weekend. I get permission first though, so if we set the alarm off (like we have done in the past - water into a door alarm sensor) they know its us and don't over react.
We start to plan this about a month before the school closes. We also need to consider holidays taken by the staff as we need a scaffolding tower erected in the main atrium area as we also clean the internals of that as well. As of last clean, only one of the staff had a 'ticket' to erect that scaffold tower as they had 2 new staff members. The staff member had 2 weeks leave during the holidays.