@jandyman Thanks for the explanation. I do a lot of two sided machining. I use 1/4" aluminum “dowels” in 1/4" holes to ensure that the top and bottom are precisely aligned. The technique is well described in both the Vectric videos and those of Mark Lindsay. I’m sure that you’ve seen that technique and found yours to be better, though.
As to 3D probing, there are commercial probes available, like this one for example
https://www.nextwavecnc.com/shop/DIGITAL-DUPLICATOR-FOR-CNC-SHARKS-AND-PIRANHA-p114869869
For DIY, you may want to look at Bill Korn’s efforts here