Don't know what kind of gun you are using, but here is some general information that may or may not apply to yours.
If you are reloading for a bolt action or single shot rifle/pistol, and use your reloads only in that firearm, you don't have to full length resize every time.
For an autoloader, you will have to full length resize, because the chambers are generally not as tight, and the cases expand a little more during firing.
Smoking the case neck and shoulder with a candle, and then feeding it into your die a few thousandths at a time, is the easy way to tell where the neck sizing stops, and the shoulder contact begins.
I set up my .223 die, by resizing a once fired case from an autoloader, that was too long to chamber in my gun (Encore).
Kept tightening the die down, until the sized cases just chambered without force. Rechecked everything a time or two, and seated the rounds in a case gauge to double check, and that is where the die has stayed ever since. If my cases grow, the shoulder will be pushed back, otherwise only the neck is sized.
Incidentally, new Winchester White box 5.56mm will not chamber in my Encore, but new Federal .223 chambers just fine. I can't measure the difference with my calipers, but there is enough difference in the shoulder position that it shows up when I try them in the gun.
Bill