Unattainable 1:1 magnification with the JJC FDA S-1 and Micro Nikkor 60 2,8

The JJC FDA S-1 does not allow a 1:1 magnification with the Micro Nikkor 60 2,8 G. It only reaches a smaller factor (about 80%. Initially, the probable reason looked like it was a fault of the original step-down ring. Using a thinner step-down ring, as it has been done in this video, does not improve the outcome. As a comment suggested, though, the problem lies in the lens and not in the equipment. The Nikkor G is longer than the D version, hence, the problem. A fix would be to reduce the length of the final tube (the one attached to film holder.)