20-30% sounds fair at first but mick is right building a round and dont forget running one can be a bloody pain cleaning it is the easy part,
i would say if you are doing totally doing nothing and i mean nothing then 20-30% fine
but some rounds need managing ,cancelations ,gates etc etc if you can setup mega days for your subbie and do all his admin ie customer stuff and pay him after work load is completted promtly (not wait for customers to pay first) and ensure your subbie gets paid outright no matter if a cust refusfes to pay then somewhere along the lines of what mick is saying also makes sence, that would be because you are still very muched involved anyway thats one way of taking a larger percentage as you will be hands on.
it also depends on the prices too end of the day whats a reasonable amount best to sit down ask them how much they wana earn and see if you guys can come to a deal! its gota be worth it for both