I would say et30 on a 8x17" wheel and 215/40/17 tyres with no arch mods,anything below and I think your gonna struggle with arch contact from the tyre.All this presuminmg you have Konis and a decent set of springs fitted to the car,from memory you do so that's what i would say from my experience from owning 4 3dr cossies (inc a 500) ranging from 17" to an 18" wheel.
I hope this helps Steve.

CheeRS.Lee.