The way I would approach it and this is JMHO...
If you are starting with new factory ammo, neck size only for 2-3 firings or until the cases become hard to extract. They are now expanded to fit your chamber. Set your full length sizing die to touch the shell plate. Begin full length sizing in small increments until the case will chamber with no resistance. This is best done with the firing pin and ejector removed from the bolt so that you are only feeling resistance from the case and not the cocking mechanism or ejector spring. When the bolt closes with zero resistance, that is your guns headspace. Measure that case with your comparater and reord that number. If you can do this with several cases and take an average, it's even better.
The measurement on the comparator is not an exact headspace measurment but rather a relative number you are going to be working off of.
Turn your die down until the comparator measures .002 shorter than the original measurement(for a bolt gun) Once the comparator reads .002" less, lock your die and from then on every case you size will be right for the headspace in that gun.
Your throat will eventually erode and you will have to "chase the lands", but the headspace will never change until you rebarrel or load for another gun.
Be sure you use the same shellholder you used to adjust the die.
If you are loading the same caliber for multiple guns, the Redding Competition shell holders are worth looking at. They let you leave the die at one setting and achieve different amounts of shoulder bump by only changing shellholders.
If you want to know the exact headspace measurement in your chamber the RCBS Precision Mic will give you that.
