I reckon it depends how you want the car to feel.

I'm on 15s with standard 225s on the rear and a 205 front.

It's a bit more neutral than with a 195 front but then I don't like a lot of

(any!) understeer.

I know you can alter it all with geometry anyway but it's tricky to get right and can be expensive unless you can do it yourself
I'd keep the gap therefore to either 20 or 30 mm depending on how you want it to behave.

R.
