I'm currently running a 18x8.5 +35 square set up, 255/40 tires and camber -2° front -1° rear. No rubbing issues until I put 5 mm spacers on all fours. Fronts were fine, rears rubbed a lil. Rears would most likely clear if I rolled that inner lip in the rear fenders.

So if you want 18x9.5 with 285s in the back, a minimum of +43 offset would probably best.

I've never seen an 18x9.5 +43 though, unless it's built to order.

 

There is a couple people here that are actually running square setups of 18x9.5 38 with 275/35 both of them rolled fenders at the rear for previous setups, so I can't say if the 18x9.5 would've required a roll of fenders at the back. Diameter wise a 285/35/18 is about the same as the stock tire, I think it comes down to how aggressive your drop is because the tire is wider