In my experience, the lens-to-lens variation in the fast FSU lenses tends to be greater than the systematic variation between the FSU cameras and well-standardized Leicas and Canons. The challenge, of course is that you need to be able to get at the film plane to do the adjustment. I have two cameras that I use as standards, a FED-2 with its wonderful long-base RF, and a Canon P, which is an extraordinarily well constructed camera. I recently had a go at a very nice J3 that was badly set up as received, and I found that the same shimming solution worked for both cameras.
It's a good idea to have one camera and one lens that you always use as a standard to individually adjust lenses and rangefinders. With FSU cameras, especially ones that have seen a bit of use, there are many things that can cause focus problems, and the tolerances in these dimensions will confuse the issue if all the variables are present at once.
The RF can usually only be adjusted at one point, and one needs to take it on faith that the RF cam profile is correct. It's easiest to set it at infinity, while using a known-good lens (I have a Canon 50 f1.4 I use for that), but probably wiser to set it at 1.5m or so.
If you are lucky, the lens when shimmed correctly on the standard camera also focuses properly on the camera on which it is to be used.
Cheers,
Dez