simply, higher number the offset-> will sit further inside from fender
lower offset number, will fit closer, and will stick out eventually.
what you want to find out is, what certain width rims would fit without rubbing the fender/qtr panel. for example, i ran 7.5wide rims with both +45 and 42.
+45, i had no problem even tucking the tire with small camber. it rubbed in the back slightly.
+42 however, rubbed every now and then since it sticks out tad bit compare to +45. also the camber was corrected with this, so i rubbed a bit more than often.
rubbing issue also has to do with what tire you run