The way I understand it, is that the higher the positive offset the further in the wheel and tire will be in relationship to the fender. In other words a +35 offset wheel will sit more flush to the fender than a +45 wheel. I believe you could even run a 10mm spacer with those wheels to get them to be more flush to the fender if that is what your after.
See the illustration below and you will understand the concept.
There are some really sharp people here that know this stuff really well, and one of them might pop in here and add more info.
Where are you Martin?