I would have gone for Neutral when taking that measurement... you are taking it from the top of the impellor housing??
Was there any reason earlier upthread you thought the shaft length might be out from doing the impellor a while back... or was that just a "probably" once this current problem started troubling you?
I assume if you operate the gearbox with the shaft the "detents" in FNR feel quite positive?
Oh well my logic says if the shaft length can be positively proved like this but doesn’t allow the link to readily align in neutral the cables from twistgrip to gear change pulley need revisiting all the way along. Perhaps now return to the idea of setting the cable adjusters so the nylon pulley rotates to allow the clip pin to align??
All done and working fine. Yes, checked thread and from all the way in there is 20 odd turns or 17mm thread. At 419 as set before then 16mm or so and I've re done it at 423 in the end so 5 turns more out leaving 12mm threaded on a 6mm shaft which I'm happy with.