superchikn

Hello everyone. Need some help please. I am changing the wheels and tires on my 2011 Ralliart .The tires I have purchased are a 235/40-18 that is configured on an 8-9 wheel width and at that are nearly the same overall dia as the stock wheel and tire set up.
The car is stone stock, it wont be ganstered or dropped, but down the road will upgrade suspension .
The wheels I have chosen are the Enkei Raijins in an 18x8.5 to keep tire diameter correct, and maybe later add more tire after engine mods.. Im thinking from what I have read on the various pages is that I need 45 offset to set the fronts flush, and yet not be rubbing and cutting tires. The 45 on the rear will allow the tire to still clear the fender but also the rear suspension.
Is this correct, or do I need a 40 offset?
Any thoughts and shared knowledge would be great.
Thanks in advance.