I have a set of 18x7 1/2 wheels with a +42 off set. If I wanted to make them sit flusher to the fender, more like a +35 offset wheel, couldI just buy a 7mm wheel spacer? Also does the 7 1/2 wheel width come into play where I would have to go with a 10-12 mm spacer to make it sit pretty.

 

Offset is the distance from the center of the wheel to the backside of the mounting surface. So to answer your question, Yes. Buying a 7mm spacer would give you 7mm out towards the fender making your rim a +35. How flush you want to make it will have to be your call. Just don't go too far that looks weird.