There is no need or reason to measure, simply adjust the sizer so cases chamber with a tiny amount of resistance,.
This is true, careful die manipulation, 1/16 turn at a time. When you get done, smoke a case with a candle or cigarette lighter, and carefully chamber the sized case after wiping the lube off the case.
You should see slight deformation of the black soot on half of the shoulder. If you do not see this, you have probably pushed the shoulder back too far.
Measuring is just easy, write the length down in the top of the die box or in a note book for that rifle with that particular barrel. Check more than one case as brass spring back is a real bitch to deal with as brass gets work hardened along with the fact that there maybe as much as 6% variation in hardness within a single lot# of brass.
