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It will take me some time to go through all this. Thank you, for the detail. I printed off the drawing earlier, and its pretty close, if not exactly, my findings.
My "about 2mm" measurement includes the rear element mount frame, as thats the surface that gets hit by the mirror and what everyone is concerned about.
One qualification is the thickness of the EOS bayonet, one starts with. Mine are 8.23mm inward offset from the camera mount face, which may be different.
Another is where one assigns the 42mm register zero point in the FD lens. I think your right on it, according to my measurements, but who really knows?
My comments here are about the FD50L and maybe should be in the FD50L thread, but I think the method is the same as and related to the FD85L, given my supposition that the rear element in both lenses is the same distance from the sensor (to be confirmed). Matching the infinity sharpness and original spacings was a long iterative process. I made the rear element in a M42 thread frame, so as to be able to adjust the placement. If I threaded the rear element in (away from the sensor) far enough to have the 5D mirror clear it, the main lens group would be closer than original spacing and be close to touching the rear element. I push the rear element out toward the sensor, and the main group would need to be pushed out (away from the sensor) for an increased space from original. I always do initial conversions with 2 lenses, using one as a reference.
I think we are on the same track.