I've now cut a decent register for my new collet chuck, but not getting the sort of runout I expected - 0.025mm 5mm from the chuck. I've marked the back plate so it always goes on the (C6) Spindle the same orientation as the register was cut, and I've tried the three different positions of the chuck on the backplate, but the runout is consistent at 0.025mm. I've a piece of half inch diameter EN8 as a test bar in the chuck.
The backplate goes onto the spindle easily, its not tight - but I can't feel any slop, so don't think the backplate to spindle register is the problem. How can I be certain?