This doesn’t seem to be an exact science!
Get it somewhere in the middle. The drive coupling should then be in alignment.
I had mine out for the winter service and found it helpful to do a dry run without bellows/gaiters first. You can then see the position better and make a note of thread positions protruding into the passenger area.
While I was stripped out, I took Axels tip and installed a grease nipple into each s/a pivot pin. The taper bearings don’t use much grease but an occasional squire helps to eject dirt entering the area.
Once you are happy with the position and swing of the arm, bearings just nipped up but not tight, swinging freely, then go for the final assembly with grease and gaiters/bellows/jubilee clips etc.