Everything finished, re-assemble in reverse order. The taps for the set screws show where everything should screw in, but make some scribe marks to indicate the positions before they get covered up by the rings being screwed back into place.
I've built up a collection of shims from buying parts lenses, adjusting J-8's, and converting lenses to Leica mount. It's easiest to use a digital camera to check focus, but i aslso have a TTL viewer for M39 mount lenses,
In short: you need to determine focus with the Rangefinder to a test object, 1meter is a good range. Use a ruler or tape measure, something that you can determine where actual focus is. Once you get the error, you can compute the increase in the thickness of the shim to correct back focus, or decrease to correct front focus.
I sent Jason Howe the instructions for the J-3, which has the formula in it.