CROSS POSTED FROM THE "CBA LOADS" THREAD:
After a recent incident with accuracy going down the toilet due to over-sizing the case, I figured there had to be a better way to dial in the case sizing die, and sure 'nuff, there is. They're called case gages, and you can buy them from Hornady or from L.E. Wilson. But since I am frugal, I made my own by boring out a little piece of scrap metal. Stick a fired case in the gage, zero your calipers on it, and that's your reference point. Now adjust the case sizing die until a sized case has the same headspace, or in the case of a hunting rifle, perhaps a couple thou less headspace.
For years I adjusted the case sizing die by trial and error, sizing just enough to ensure reliable chambering. However, the trial and error method depends on "feel" and is subject to human error, as I learned the hard way.