this is an interesting one, I'm in similar position, taking him on 1 day a week. was going to pay him around 80 quid, but thought about paying percentage as incentive too,
was thinking around about 30 percent of total income, or just over, not too sure how much would do together but guessing upwards of 260, which would put him around the 80 a day or better if you pushed on.
guess u need to work out how much likely to do a day, how much he wants/u want to pay, and work out percentage based on that?