Respectfully, I suggest you are over thinking things. There are two ways you can attack this.
(1) you utilise Leitz's alignment tooling. Assuming that is not available
(2) you will have to dial the calibration in by trial and error.
The only way you can realistically have any hope of obtaining decent accuracy will be by removing the body casing, and using a ground glass and loupe across the film rails to compare what is actually transmitted through the lens, with what you can see through the viewfinder. Tedious, but this will work.
It really doesn't matter if your camera moves in between your adjustments. As long as you have a means of returning it to an identical position every time, it will not affect the process.
I would suggest that the close end is likely to be the most problematic, it's where parallax will be at its most extreme. There are no guarantees you will not have to revisit this—but, if I was doing this, I would: Start off near the minimum focus distance, match the top left marker (it's the furthest from the lens axis, and, hence, likely to matter, the most) to what I can see on the glass; Set the lens to infinity and, 15 feet; See if I could live with any deviations at these distances and, if not; Balance the trade off as best you can.
Obviously, you will need a way of consistently returning your camera to the same location between adjustments. For a close range test (say I am dialling the viewing lens of a TLR in to its taking lens and comparing the close range match) I will often point a camera at a black and white magnet on the front of my white refrigerator. Lots of contrast helps nail the focus.
If you need to do a frame calibration, some strips of black auto electrical tape on the door of a white fridge for a reference target will serve the purpose nicely. How to lock in the camera position? Well...place two or three tape strips onto your kitchen bench against which to line up the front and sides of the shutter crate between tweaks. Or some other stable means of support. Quick and dirty, but good enough.
Comparing the infinity framing means pointing the camera outdoors, but, again, surely you have an outdoor table, window ledge or even a car roof onto which you can consistently position your camera sans casing. Watch the car paintwork if you resort to that.
Unless I'm mistaken, in the attached pages from their manual, Leitz advise to base a calibration on the 9cm framelines. (The longer focal length is likely to be less forgiving of parallax, so presumably this is germane to their instructions). It stands to reason you will want to have a trustworthy 9cm screw mount lens at your disposal. Out of an abundance of caution, I'd suggest it best to attach a Leitz 9cm of some description to eliminate possible complications such as variations from Leitz's standard focal length tolerances etc which might (or might not) present themselves if other makers' 9cm optics were attached.
Be aware that extricating a IIIg from its casing is somewhat more tedious than earlier screw mounts. Well I imagine it is—I prefer the IIIf and IIIc models, personally. But I have fully serviced a IIIf RD ST and, whilst the timer escapement itself need not be disturbed—the various shims, distance pieces and other components which comprise the overly complex connection to the gear train can be extremely tedious to reinstall, if you have never done it previously. (Having said this, one of our members owns a de-timered IIIg; perhaps it's you?).
A soldering iron should be needed to disconnect and reconnect the wire for the sync terminal. You'll need to have the tools required for the fastener under the rewind knob and, possibly the bezels for the RF windows and so on.
For all this the screw mount models are, generally, a delight to work on. They have everything they need to make quality images and (IIIg excepted) nothing they don't. I did a full service and curtain replacement on a friend's IIIf black dial a few months ago and it all went fairly smoothly and the camera is now returning accurate times and is back in regular use.
Might as well clean the finder optics while you're in there. Watch the rear surface of the beam splitter! Leave it be or dribble lens cleaner across it with the body upside down. They can usually stand some limited direct contact, but...better not to touch it, if at all possible.
Obviously up to you as to whether you can manage the disassembly and reassembly process. If in doubt may be better to solicit the services of an experienced technician. Mind how you go.
😉
Brett
