Richy
I don't want to be a party pooper, but the 338 customers you require, is that figure taking into account all the ones who will mess you around by saying they want a regular cleaner, instead of saying they just want their windows cleaned.
Depending on how you will be getting these customers, this will affect the cancellation rate, if I was you I would increase the customer total up to 428, and spread the time to 26 weeks.
Also how are you going to run the vans, are they on a 3,4,5 or 6 day week?