It can be pretty challenging. For your set up you you've got a lot going. I'll try and lay it out in steps.
OEM wheels are 46mm offset and 7 inch width.
Your wheels are 8.5 inches and 43mm offset.
46mm - 43mm gives 3mm offset outward.
The width increase adds 38mm of width over the 7 inch which is divided in half. Half inboard and half outboard or 19mm each way which moves the inner rim in by 19mm.
You already got 3mm out minus 19mm in gives you net 16mm inward increase.
The 15mm spacer moves you outboard again giving you a net inboard offset of 1mm which is nominal.
With the inboard side established back to about the normal, you've also added 19mm outboard (the other half of the 38mm) which closed up the gap between the tire and the fender.
Net of all this, you need the 15mm spacers since you can't go inboard any. Fender roll and pull or dropping in tire width to 215/40 should fix any rub.
2006 Z4 3.0si Sport Package (But I'll still hang with my "homies" in the E36 forum)