[Clark]

I buy between sizes pin gauges that fit in the bore from MSC.

I make a gimbal from heavy Copper wire in the 4 jaw.
I adjust the spider where ever it needs to be to get the pin [spud] that is just to the right of the gimbal to be concentric and parallel with the spindle.

I do the same for the crown and the chamber.

Here are the two pin gauges I use for .223 and 6mm. They are ~ $3 each.

With an Intrepid .0001" test indicator I can get the bore very close, but I no longer believe that concentric chambers have anywhere near the importance of concentric ammo. Ammo can be chambered with any rotational orientation, but the chamber error is always the same rotational orientation. I chambered a 7mmRM .005" off center and it shoots great.