With an 18 x 9" rim I would not vary much from the OEM 34mm offset. There is not much clearance (12mm) in front between the tire and strut/spring. I went with a 40mm offset on a 17x 8" Beyern rim and have very little clearance left (6mm). So far I have not had any problems
Use the calculator above and input the specs on the wheels you are interested in and measure your clearance with your stock setup so you know what you started with.