Got carried away with my "cute" reply and missed the second part of your question. My generally accepted way of working with headspace on the mags/reloading is.... new brassfire them to the chamber and I bump them a couple thou from then forward. With used brass it's a lot more variable.... I try to find a cartridge that will only chamber with a little effort, not shaving brass tight! I then bump it down a thou or so at a time till it chambers smooth and use it then as my "master" cartridge. I'm assuming in all this you have a way of accurately measuring the cartridges like the Hornady kit.
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