When loaded, my hexagonal collet block rests on the threaded end of the collet. I was surprised to see how far it was off-square & decided to rectify this. Grinding the nut flat and adding a ground spacer, the collet block now rests on these two ground spacers and sits perfectly square.
Once this issue was resolved I made a ball end vise end stop that located the centre of the collet block exactly 150mm from the stop outside end, including the edgefinder.