If you are only neck sizing, and you already have several firings on your brass in your rifle, and the bolt still closes easily, then chances are your shoulder is right where you want it right now. The idea is, to have the case fill the chamber completely, but not tightly. So when your cases start to cause drag when you close the bolt, or even won't close at all, then it's time to FL resize, or at least to bump the shoulder back with something like a bushing bump die, which sizes the neck with a bushing and pushes the shoulder back without resizing the body any. I have both, and have settled on FL resizing due to case head expansion issues, which can also cause difficult chambering.
This is where the measuring takes place. When your brass becomes difficult to chamber, measure the case's headspace with the appropriate tool. I like this one but caliper-mounted tools accomplish the same thing:
http://www.midwayusa.com/product/574...308-winchester
Once you have this measurement, record it. Check several similar cases for a consistent measurement. Now take your FL die and set it so that it just bumps the shoulder back, barely. Hence the .002". You will be able to measure this with the appropriate tool. Again, record this #. This is your size-to length. This will allow easy chambering again, without needlessly overworking your brass. Do this by starting with your die backed out a couple of rounds, sizing a case and measuring, turning the die in a little at a time, snugging up the lock ring each time, until you get .002 "bump" of the shoulder. Check for repeatability with a few more pieces. If you're getting a consistent .002" less than your previously recorded max length, you're good.
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