Remarkable and well articulated answer Phil. I think all can benefit from that answer, but if it is a bit much for someone I would go after trees answer also, not rocket sience but will get you though the task. An answer like trees can often lead to refiguring costs and possibly having to raise the price later, which does not always make the client to happy. Phils approach will get you a lot closer and allow for a decent margin.
The actual math, once you have decided the hours, will be pretty simple. If I was in your position I would, and do, figure my wages on a 57 week basis. This allows for a portion each month to go towards Holiday cover. You will have to decide what is an acceptable margin for you by the size of the job. As phil was saying, ususally, the bigger the job the less margin you can expect if you want to be competitive. Dont forget that part of the margin will go for materials (chemicals, equipment, etc). If you allow 3% - 5% you can usually get it about right. If you want 25% for you and allow 3% for materials and you estimate it to be a 15 hour weekly job that you will pay 6.00 per hour the math would look close to this. (((15 x 57)/12) x 6.00)/.72 = 593.75 per calendar month.
Your requirement for margin may be higher or you can get by with less materials or the pay rate being higher will change it a bit. but the math is pretty solid.
Regards
Gilbert