Got myself one of the doo dads!
Let me see if I got it right;
(assuming all set up), insert fired cases, measure longest base to shoulder case. Use that one. Note measurement.
Screw in FLS die until it bumps back the shoulder to the above measurement less the clearance you need (1-2thou for bolt rifles, a few thou more for pump, auto rifles). Once you hit this you are good to go, cases should be resized to allow 1 thou clearance from shoulder.
Am I on the right track here?
Surprisingly there was only one video on YouTube about this and the guy spoke so softly if was hard to fully understand.
It occurs to me that if you are about to head off and hunt in a hostile, dusty environment, an extra thou or so would be a more reliable setting.