Even with et38 you'll probably need 5mm.
I am gonna fit either a porshe or Ate BBK as soon as the cash arrives ... I have et45s at
mo and will have a 16mm spacer! Thats equivilant of ET29-30 whereas et38 with 5mm will be equivilant to et33!
Most manufacturers will just add a little extra to the back of the hub (machine a little less off) to reach the bigger offsets such as 38. So this essencially does the same as fitting a spacer to a smaller offset. Not sure if all do this but it could be your best option would be to forget the spacer and just fit a ET30 wheel? Have a word with roland if I were you.