I use cartridge case headspace gauges to set up my dies.
the difference between the top and bottom notches is more or less the maximum amount of stretch a case can take before case head separations are a problem.
An example of too much clearance between case shoulder and chamber shoulder
Comparators work well, and as for my 7.5 X 55 Swiss rifles, are the only device on the market which I can measure case length changes before and after firing.
I recommend don't size to zero case shoulder to chamber shoulder clearance. It does not improve accuracy and instead will create problems in the future. You will encounter hard chambering, and hard extraction. There needs to be clearance between the case and chamber or the case will be compressed by the bolt lugs and the chamber walls after firing. The lugs and chamber are steel, they are operating below yield, they both stretch under the pressure, but relax to original dimensions. But, steel and brass relax differently, (brass is in fact operating above yield) and interference fit brass cases will at some point be under compression from the lugs and chamber after firing.
I recommend setting up your sizing dies so the shoulder is pushed back 0.003". There will be small variations in a batch as cases work harden, therefore there will be some where the shoulder will be set back somewhat less than the average. Three thousandth of an inch is enough that a little bit of unburnt powder won't cause hard bolt closure, and my cases lasted forever.
Shoot enough, I am certain you will come to the conclusion that reliable feed, ignition, and extraction are more important then theoretical tiny improvements in group size. While bench rest and F Class shooters desire the ultimate small group, they are still extremely careful to create ammunition that will feed and extract. You get a limited amount of time to shoot your string, having to knock out cases with a cleaning rod will ruin your score, as the wind will change during your down period! (The Range Gods are watching, waiting, for an opportunity to mess with your score!) My F Class buds don't feed rounds from a magazine and many don't have ejectors on the rifles. I do not recommend F Class reloading practices for anything except F Class competition.
I did have to go to a zero clearance case shoulder to chamber shoulder clearance with my 35 Whelen's. The case shoulder of a 35 Whelen is shallow. In some of my rifles I had misfires and hangfires till I installed a stronger mainspring and went to zero clearance. I am sure the shallow shoulder on a 35 Whelen cushions the firing pin blow. I do believe the sharp shoulders we see on the current F Class cartridges are there to maximize the energy delivered by the firing pin to the primer. Better and more consistent combustion.