A lot of this depends on what camber you run and how you get that camber (top plate vs at the strut mount), but 18x9.5 you'd probably want +25 or less offset, ideally +20 or so for a 235. I run 225/40 +21 and easily have clearance, but I'm right at the fenders when aired out. There's a bigger airlift thread
here with lots more info.