tire to fender lip clearance

Guys and Gals, how much clearance is needed between the widest part of the tire sidewall and the fenderlip on the rear of a car? This is a leaf spring with rubber bushings 68 dart with no rear sway bar. It will be a low profile tire 45 series tire. I want no rubbing at all. I have read lots of posts and searched for a good solid answer but have not been able to find the post with the answer. I am sure this has been hashed out before and is somewhere on this site but I have not found it. Is 1/2" enough, is 3/4" needed, or maybe an full inch is required. I am curious.