Appropriate Wheel Offset for 18" Wheels?

I just placed my order on wheels. I'm wondering if I should order 245/40/18's and start by there or just stick with the 255/40/18's and fiddle around. Remeasuring, I think I'm going to need some small spacers but as long as I have good thread engagement, I think I should be fine. I can't afford to have the wheel hit the frame rails due to brake lines.

Find tires on craigslist in that size used and cheap to test fit and if they work but new ones.
That's what I have been doing
Much cheaper than buying new that don't work