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Any 17" rear wheel shall rub the lower control arm at an "actual" back space of 6-3/8".

A 3 piece wheel having a calculated backspace of 6-1/2" has an actual backspace of 6-1/4". The calculated backspace allows for 1/2" thick tire flanges, whereas the tire flanges of a 3 piece wheel are actually only 1/4" thick, hence the difference between calculated and actual backspace.

6-1/2" calculated backspace is 1/2" offset (12.7mm). At that offset a 335mm tire sticks-out beyond the edge of the fender by 5mm.
Those dimensions are based upon actual measurements I made about a dozen years ago on the 17x11 Kinesis wheels that were on 6018 when I purchased it. Although they weren't Pantera East campy clones, I doubt there's any difference as there just isn't any wiggle room with the 17x11 wheel. More offset would cause rubbing, less offset is going to hang the tire outside the fender even further than 5mm.

David Nunn could verify.

You're 100% correct. 6.5" backspacing (ET 13) is the optimum offset for a 17"x11" with a 335 section width tire. It just isn't safe to go further inboard and it will stick out a very small amount. If you go up to an 18"x11" you can go further inboard because an 18" rim clears the obstruction that interferes with a 17".
An alert that more backspace on the rear wheel has a consequence. When the suspension is under compression, the tire will rub the upper frame rail sooner. A good precaution to reduce risk of damaging the tire is to smooth the welds of any sharp surfaces. Another approach, if you have aftermarket coil overs, is to remove the spring and reinstall the shock. Jack the tire until it strikes the frame rail and measure the exposed piston shaft length. Then, go to a site, such as RE Susupesion ,that sells numerous size (width and length)and hardness shock, bump stops to select the appropriate stop. This is a terrific site in that they have graphs showing how each bump stop behaves under compressive force.

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