Like any of those gauges, it will only be as good as the micrometer you mount.
When I was shooting HBR, I made up a unit very similar to that in your post. I was lucky enough to have access to three different dial micrometers.
Mitutoyo, Starrett and a Princess Auto knock off.
The Mitutoyo was the best as it was smooth as oiled glass through its entire length of measurement from dead zero to just over an inch.
The Starrett was almost but not quite as good and had less smoothness of movement but very accurate as well.
The PA knock off worked but was a pain to use as its movement was scratchy and the indicator needle would bounce constantly.
I see the ''spud'' is made from aluminum. I made mine up from 4140 steel and polished the surface to its final measurement.
I liked a round spud, that was a few thou smaller than the expander balls or mandrels in sizing/decapping dies.
I also had a couple of spuds that were made from the pivot knives on large balance beam scales that were triangular on one end and extremely hard. Couldn't touch them with a file so a small die grinder and polish wheel were used to round off the balance edge and just fit into the case mouth. These were OK but not as good as a round spud with a center rod and decapping pin as shown in your link.
The neck should be turned over a tightly fitting spud, which has been machined and polished to exactly the same diameter as the bullets you bill be shooting to keep everything absolutely concentric and consistent.
I keep the mandrel spud chucked up in the lathe and use the tail stock spindle with a pointed tip in a drill chuck to force the case mouth over the lubed mandrel before turning to desired thickness.
If you know your exact chamber dimensions, it's very easy to turn the neck thicknesses so that you only have .002 total clearance.
Some folks like to have .002 clearance all the way around but .001 all the way around is fine. I wore out a couple of barrels with case necks turned to those dimensions.
Those hand held units are OK but not great for serious work.
