this website came in handy, when i was prepareing my Fuchsfelgen project...
1010TIRES.COM - Wheel Offset Calculator
In the end several factors determine the spacers you need...
wheel size
tire size / brand
suspension type
As far as i heard, a tire size smaller than recommended for the wheel is kinda "okay"... a size bigger than recommended "troublesome".
Mine:
Front 8J x 18 ET52 --> 5mm spacers = ET47 (within oem range) --> Conti 225/40ZR18
Rear 10J x 18 ET65 --> 18mm spacers = ET47 (within oem range) --> Conti 265/35ZR18