I have E89 wheels on my E86, so with the different offsets I've effectively got 18mm on the front and 10mm on the rear.
Our forum wheel guru, mr wilks, suggests 18mm on the front is a bit marginal so 12mm or maybe 15mm would be good - although I haven't had any rubbing so far. But mine could easily take more than the 10mm it has on the rear, so davros' suggestion of 20mm sounds good!
Mine is also on standard Sport suspension.