WEll, i just installed GC coilovers and i just got new wheels and well i found out the fronts rub badly...
my setup:
18x8.5 with 245/35 eagle f1's ET40
I want the smallest possible spacer, i dont want the wheels sticking out too far out.....should i just get the 12mm, how exactly can i measure the spacer i need....any advice would be appreciated

I'd just get 5mm.... I mean come on, if 5mm doesn't fix your rubbing problem (I am presuming that it's rubbing on your new suspension), then there is seriously something wrong. 5mm I think would be enough. If you move your wheels out half a CM and it still rubs well then it must have been almost unmovable from the get-go...