Re: Correct Offset for Z4
it's difficult to answer your question without knowing what's the width of your wheels.
the stock 18x8 front and 8.5 rear offsets are 47mm & 50mm respectively & there's plenty of space between the fender. If u want to use size 18x8.5, I would imagine ET 35-40 would be nicely flushed against your fender, anything lower it will rub.
check with tuners like bbs or AC for recommended wheels for your Z4, it will tell you basically the ideal offset for your car