None of my window cleaners are on hourly rates, they are all on %age of turnover, if it rains they get nothing, however if the sun is shining and they are working hard, they get very well paid.
It's swings and roundabouts really, you have to decide which way you want to work it. If you pay them an hourly rate then you will generally find that the work they get through in a day is a lot less than if you pay them a %age of what they do.
All of the lads who work for me prefer it this way.
As for holidays, the guidelines from the Inland revenue is that they get an average of the previous 12 weeks earnings, which for one of mine was over £70/day for his holidays next week.
This all has to be factored into your pricing when you take on new work