Re: Headspace guage
"I am assuming then I can just use the fired case to set the sizing die? Measure them, and then adjust the die."
Exactly. Fact is, FL sizing pushes the shoulder forward as it squeezes the body diameter down. All we need do is restore the sized shoulder to match the fired dimension and you will be able to chamber the round but will have minumum slop in the fit.
Sizing this way, correctly, reduces case stretch to virtually zilch and head seperations will be rare. I have cases I've reloaded 6-10 times with no sign of a seperation. And I don't load mild.