A 30 offset on the front puts the rim and tire in the exact center of the space available. A 245 was perfect but I wanted to try a 255. With a 255 and 30 offset I had a tight fit with a credit card (1mm) to the suspension bolt and I would hit the inside of the front fender on big bumps or under heavy braking. 30 is where you should be trying to end up IMO. I filed the nut and arm just in case and rolled the fender. With a 235 you shouldn't have any problem with a 10 or 12 mm spacer. A 235 isn't wider than a 9" rim. The rear, I think you need a 15 mm spacer. I'll take some pictures and measurements when I can but really, if your car is like mine I don't need to. A 285 is a little, but not much, wider than the rim. Well, in an 18 anyway. Borrow the 10mm spacers, you'll see.

Les