A few points.

Cutting the spigot the correct size should leave it perfectlly concentric when fitted to that backplate, then only requiring a dot punch or scribe mark to be sure to remount in exactly the same position if it is demounted for any reason.

Don’t use calipers for more than rough’n’ready internal measurements. There are better ways (even if you use the calipers instead of a micrometer)

You ideally need to have an undercut at that internal corner, to ensure the faces meet without interference, although the chuck body may have been chamfered sufficiently.

You need to fit the collet before the cutter/bar/workpiece. If the collet has not been fitted properly (a risk – it should not just fall out). If it was not fitted correctly, your measurements may be fortuitous.

The correct torque needs to be applied to the collet nut to achieve the run-out spec.

The claimed run-out is only applicable for the maximum size quoted for the range of any particular collet.

Tip: remove those two inner lugs from that collet nut spanner. Sooo much easier to fit in place. Think here that C spanners are very easy to fit – they only have a single lug.

Edited By not done it yet on 07/07/2021 09:23:30