Never think about an hourly rate!
Price up per window and you get a much better uniformity of price.
When you price by the hour, the faster you get, the more work you get done and you don't benefit from the fact you have speeded up.
There is a whole lot more I can say about pricing, but it'll take too long.
But there is a difference between the hourly rate you eventually earn, and you can only truly work that out when you do your accounts at the end of the year and remove all of your business running costs and so on.
The amount you earn per minute worked is a very different animal to what you perceive as your hourly rate...
Break the work down into single units of standard window size and charge a set amount per window, £1.00 per window, £2.00 per window, whatever floats your boat, allow a little more as you go up higher though.
Once you have done that you may have to add a little due to access and parking and so on, or perhaps traveling.
Ian