Like others have said, case gauges are ok but a bump gauge is the way to go. I prefer the Sinclair comparator body and respective bump gauge for the caliber I am reloading. I measure a once fired case, fired in my rifle, and then set the die to where it full length resizes and only bumps the shoulder 0.002-0.005 back. That'll give more life out of the brass than trying to duplicate new rounds that are sized to fit in all chambers.