Rear Tire to Fender Clearance

[35spline]

I would like to see some opinions on what is the optimum amount of clearance one should have between the outside rear tire surface and the fender. I have my wheels and tires mounted and will need to measure to narrow the rear housing. I would like to keep the rear as wide as possible without worry of any rubbing issues. I will be locating the rear with a panhard bar and the vehicle is a 1968 Camaro and I have already trimmed the inner fender lip to a minimum. I came away empty using the search.
Martin