I have a set of SSR SP1 wheels on my MkIV. The fronts are fine in offset but the rears (18x10.5) are a bit conservative for my liking (+55). I was thinking of getting some hubcentric spacers (either H&R or Ichiba). I have 305 Nitto DRs on there at the moment but intend to drop back to between 275 and 295. The noob questions I have are: would a 15mm spacer be equivalent of having a +40 offset (this simple math seems correct to me) and would this cause any rubbing issues (assuming I am dropping back from a 305)? Thanks.