Hiya Gary,
I have met several window cleaners who rent out their rounds, the percentages vary greatly.
Figures of 15-50% have been mentioned, obviously the lower your rate, the less likely it becomes that the guys working the round will take it from you.
Also it is much harder to steal a round, if it is WFP, as there are the additional costs of their own WFP and van.
However, you will obviously need a watertight contract, that mentions how much value per month is in existence on hand over, and any drop outs replaced,or you charge the w/c concerned x4 from their wages if they dont replace that lost value, when it is eventually handed back.
Reward the w/c for any new business they bring on, that way they should always announce a new customer to you, ensure they get their insurance as it is much cheaper than you yourself getting it for them (£80 if self employed and £800 if employed!)
Good luck with it anyway!
Paul