There are so many variables here, what do you call a full round, what earning potential are you expecting from him every hour and how many hours per day ?
Say six hours a day - 30 hr week - and gives you a production rate of £30 per hour £180 per day
From that his hourly rate is £10.00 is £60 per day but then you have to cover 28 day paid holiday plus employers contributions and insurance this works out somewhere around £18ph (£108) then the cost of running and insuring a van, costs of making water etc, etc... £25 per day giving a total of around £133.00
Leaving you £47.00 to do the admin, customer service, finances and give you a living wage
(If I understood the employee is doing all the windows while he does pressure washing)
Of course if they can punch out £50 p/h every hour then the dynamics change. Same as if Ian was taking on a man and they did the round as 2 man until such time they did a round each your earning, then his bit is extra.
Hope this makes some sense
Darran