It is not necessary to use a headspace gauge to set a full-length resizing die correctly. Brass may "grow" slightly in length with a properly adjusted FL die, but that is because you're moving metal when you size the neck and body of a fired case. The good news is, that is why the case trimmer was invented.
To the OP, you may find the shoulders move on your commercial brass after the first firing anywhere from about .005" to as much as .008" after the brass has expanded to fit the chamber. It is only necessary to knock the shoulder back about .001" to .002" to easily fit back in the chamber. If you continually knock the shoulder all the way back to the factory length, you may well start to see case head separation after a certain number of firings, usually far fewer than it's life should be. That is because you're compressing and over-stretching the case wall more than necessary. It will soon start to give at the weakest point, which is about where it thins out into the case wall proper, just above the heavier brass of the webbing region.
To set your resizing die, first read the instructions that came with it. In general, if you screw the die body all the way down with the ram in its uppermost position, you'll end up with the shoulders knocked back very close to where they started out as virgin brass. Many dies use 7/8-14" threads, which means there are 14 turns per linear inch of movement as the die is turned. If 14 turns moves the die body one inch, it is easy to calculate how much of a turn is necessary to move it ~ .001", or .005", or any amount you desire. For example, (14 turns/1.000") = ("x" turns/.001") >>> solve for "x", which comes out to .014. Deciding how much of a turn .014 is equal to is not so easy for most, so you just multiply by 360 degrees (one full turn) to get 5.04 degrees. So you need to turn a 7/8-14 thread die about 5 degrees to get .001" linear movement.
Now you can measure the cartridge base-to-shoulder (CBTS) distance of a few unfired rounds, and a few fired rounds, takes the average of those measurements, and determine how far the shoulder is moving (on average) after firing. Screw your FL sizing die all the way down, size a couple pieces of brass, and do the same measurements (before/after) to determine how much it pushes the shoulder back. If it's more than .001" to .002", you'll need to back the die out by about 5 degrees (from screwed all the way down) for every .001" you need, so that you're only moving the shoulder back a total of about .001" to .002". This is not rocket science; you may have to play with it a bit and resize several cases to get it set where you want it, but it's not a big deal, those cases will still be useable. Once you have the die body set where you want it, you tighten down the lock ring and you're GTG. With the lock ring sufficiently tightened, you then use the lock ring (not the die body) with the lock ring tool to tighten or unscrew the die from the press. That way, you're not messing up your adjustment when putting the die the press, or taking it out. I find the most difficult part of the whole process is if the die body wants to turn as you're tightening down the lock ring after you already have the die set properly. You just have to be cautious that tightening the lock ring doesn't turn the die from your desired height setting. Not a huge deal, but something you need to watch. If you're using dies with a different thread than 7/8-14, you simply change the calculation above to reflect the thread pitch of the actual dies you're using.
Once you have set your resizing die in the manner I described above, if you use it to size virgin brass, you will find it does not even touch the shoulder; it only sizes the neck with whatever size bushing you have in it. So dies set up in this way can also be used to prep the necks of virgin brass, which may not be very consistent straight out of the box.
