The fact that the mount is empty does not change the fact that you have to place the Summar at the register distance to focus at infinity and some millimeters longer to focus at shorter distance. As a SLR is thicker than a Leica LTM, I doubt your the experiment would be useful.

At the opposite, Kiev88user preconisation to use a ground glass at 28.80mm distance makes sense. Instead of using a cardbord, you can use some macro rings to achieve the 28.80mm distance (this is how I check the real working distance of my old and non-standard russian lenses).