Bill,
Link below to REAL OEM parts shows the OEM offset:
http://realoem.com/bmw/partgrp.do?model=BU52&mospid=49303&hg=36&fg=15
From ET47, rear ET50
Second link shows you how to calculate the effect of varying the offset:
http://www.1010tires.com/Tools/Wheel-Offset-Calculator