rear offset 18x10

[autodoctor911]

I know this is kind of an old subject but I just recieved my new wheels from forgestar, and I am disappointed to see that they don't have as much offset as I was hoping for. I am trying to get them to fix it because I want to run 275/35 tires and a lot of negative camber, which pushes the wheel outward. I am afraid I am going to cut the tires. I measured the offset to be just under 40mm, if my math is right. I used a straight edge to measure backset at 7.12 inches, and the wheel lip to be .50 inches, so 7.12-.50-5.00(distance from inside of lip to 0 offset)=1.62=39.xxmm. correct? There is no marking on the wheel that seems to correspond to offset. please let me know if I need to send theese wheeels back. I have to be able to run these tires to be competitive in my autocross class, and I don't want them to be rubbing, or sticking out even. seeing as porsche has a bulletin that states the rear camber should be adjusted less negative(within specification) to achieve proper tire clearance when the stock offset is 47mm and stock tires are 265/35/18 I am thinking I am going to definitely have clearance problems. anyone out ther had a rear wheel/tirre combination that would have similiar offset? do you have any issues with tires rubbing?