Can I run 305/645/18" Hoosier Slicks on 997.1 GT3

[garyever]

I'm switching to 18" Hoosier slicks all around on my 2007 NB GT3 from 19" street/ track Michelin cup 2. The fronts look like they will not hit the fender ok but 305/645/18" on the rear looks like a close call as to whether they will rub. I see a # of posts on Reenlist that say its ok, but I'm not sure the math adds up. The 18"s put the square shoulder of the rear slicks 2 1/4" below the fender measuring straight up from the hub. Has anyone tried this fit on their NB GT3?