[RB725]

getting ready to make a a purchase wanted to talk to some knowledgable about the s14 chassis, Rims in mind XXR 526 18x10.5 +20 on the rears, im trying to run at least a 265 or 275 tire with hopes of fitting a 305 . I used the wheel calculator and according to this it states inner clearance 1 MM more, im assuming that means I need one mm more as in a spacer?? and for the outer extend an extra 39mm. Im a little lost. Can someone shed some light on my situation. I wanna run some wicked wide tires but not hella flush bullshit or tires sticking out of my vehicle by 2 feet either. If fenders need to be rolled sweet otherwise what do I do.....besides choose different wheels!!