You don't 'add' tax to your quote unless you are VAT reg!
Working out what you want to earn and how to charge accordingly is not really an easy thing, and although you may charge £20 for a house, maybe the same as the next window cleaner, that guy might knock it out twice as fast as you, so although he is charging the same, he is earning more money (pro rata that is)
You have to take into account how many hours on average you will work each week,and that isn't as easy as it sounds, you may be out for 8 hours but you won't have worked for 8 hours, you have remember that you only 'earn' money when you are actually cleaning windows, you don't earn any driving to and from various accounts or sitting in the van because it's pouring with rain or you are having a tea break. Or the customer is chatting to you...
Then you have to add in all of your business expenses (much bigger than most realise)
At the end of the year you only pay tax on your profits, and at the start of the year you'll have no idea what those are going to be.
Ian