You can't legally employ him on a self employed basis, if you get shopped then you are in trouble.
Having said that, if you are doing it that way then you need to do it on percentage basis or have some other form of incentive.
A 75/25 split in his favour is a real no-no, it should be more like 50/50, you have to cover your overheads, you had to set him up, not cheap to do so with WFP, and running costs have to be considered.
If we are to assume that the £12 house is a relatively average kind of semi, and the work is reasonably close together, done WFP you should comfortably be able to do 4 houses an hour. On a busy day, with good weather and so on £150-200 should be achievable.
I know that 4 houses an hour at an average of £12 per house is £48, but that won't average out on a full day at that rate, the lad may be out for 8 hours, but he will realistically be actually working for no more than 6 of those hours, but he should be able to clear £30 an hour.
Ergo, if your round is priced well it should be reasonable for him to return a turnover averaging around £180 a day. (ish)
The problem with employing in this manner of course is that the person you are employing sees you getting half the money and thinks it is unfair....but if he is working well he is earning himself approx £15.00 an hour.
If you are employing someone legally then his daily turnover will need to be at least that to cover employment costs, profit, investment, overheads and so on.
You need to kick ass!
Ian