My experience is same as past few posters. I have been playing with some non-standard cartridges - no gauges available or needed - I do not measure anything. I set parent brass into shell holder - that one has neck much too large for what I want to end up with - set the full length die what I can see is much too high - run a case through - take that case and try to chamber that empty case into the chamber in my rifle - bolt will not close. So turn in the die a 1/4, 1/8/, 1/16 of a turn - re-lube and resize that case - repeat until the bolt does close, whereas it would not close 1/16 or so of a turn previous. Tells me that newly re-formed case is snuggly fitting and "headspaced" into my chamber with the bolt closed. At that point I do not know or really care what the "headspace" measurement is - I know that brass snuggly fits that chamber in that rifle - there will not be any "case head separation" possible, with sane pressure loads.
As mentioned above - with a case previously fired in your rifle - colour up neck and shoulder on a fired case with a Jiffy marker - then again start with die set in press too high - run that case in - when you pull it out, you will see where die has scraped that marker off the brass neck - shows how far you have sized down that case neck. If you can re-chamber that brass into your rifle, then want to bring that scrape mark about 95% or so down the neck - do not need to go all the way to the shoulder. If that brass is too tight for your preference to chamber - will want to "bump" that shoulder - so a little bit more - to just so get marks on the colouring that you did on the face of the shoulder. Nothing was ever measured - you use your chamber in your rifle as the "gauge". Has all been mentioned in other posts above - maybe another version makes sense to you?
Be aware the "tight to chamber" can be caused by other than your shoulder dimension. Some chambers are larger diameter, at the rear, than what your sizing die can do - so case feels tight - sometimes can not even go all the way in - but is hanging up at the rear end of the case, not that the front. Nothing to do with "headspace" - often will need a Small Base sizing die for that chamber. Or something. Can discover the issue again by marking up case with jiffy marker and look for where it got scraped off. Tells you where the "tight" is.
Maybe I am old, maybe too fussy, maybe just doing it wrong, but I re-set the sizing die into the press - re-chambering the empty cases into my rifle - every time that I sit down to reload for that rifle. After a bit, does not take more than a couple of minutes to do - then I know that die is properly set for that chamber, and proceed to do up the rest of the batch - and I do periodically check to make sure nothing has moved on me. I have no experience with progressive presses - still putter away with single stage press for sizing, powder volume thrown onto the scale pan, trickle up powder to "0" on the beam scale, find the lands for each batch that I am doing up, and set my seating die to get the "jump" that I want - I redo that every session...