Bart B like Guffey gets all bent about head-space and all it does is cause more confusion (My opinion of course).
So, once you fire the round, you have conformed to your chamber (it shrinks just a bit so not exact but close)
Using an FL die, you reduce the case a bit until you get to the shoulder.
The goal of minimum bump back is to reduce or stop the stress on the area above the BASE that cracks by a FL and major shoulder move back.
A lot has to do with your chamber. If its a really long WWI 1917 and I go with the die mfgs (upwards of 1/4 turn) I take that formed to the chamber case and I really cram it back.
While not as bad, the target rifles are similar, its way too much push back if you go with die mfgs setup. The area above the thick part of the case at the base will see that stress and crack.
So, the idea is to bump the shoulder back enough its free bolt close ops and no bolt Resistance on closing but not so much as to crack the case at the base in 5-10 reloadings.
I got for a bit more than most do (.003) as my setup has some wander that if I don't I get the occasional tight bolt close.
So far I have yet to crack a base since I started doing so. Cracked cases are dangerous and more so in old rifles but having seen a modern file blown all to hell I think all are bad.
There is some discussion contention you have a better fitting round that is more accurate. No clue on that. I don't shoot good enough to tell.
Some contend you want a bit of bolt resistance ie. tight. No input on that either, I don't like it as its a wear aspect for the bolt lugs I don;t want to get into.
And personally, unless we are working on an engine and we are looking for really small tolerance of fit, being super technically accurate detracts from what the goal is of getting the ammo to a specificity condition.
I don't think it lends to the discussion and detracts from the method which is what we are after. If you can follow a recipe do you have to be a great cook?
Sure, American Test Kitchens explains whats going on and why it works, but if you follow what they tell you do do you get the same result without brain cells being hurt.
Maybe you go onto be a great Gun Chef and maybe not. I have worked into an understanding (my opinion again) without being bent about it all. I have the right mental picture of whats going on. If I don't, so what? I bump it back .003 or .002 or .001 and get the results I am after (bolt closes easily) then I am good arn't I?
Me, I get good re-loads (just ask me) and I am happy with the results (how well I shoot is a different story)
So yea, I get E=MC2, but I sure would not want to do the math (can't actually) - but yea, squash enough Uranium (etc) together fast enough and it gets a nasty mushroom cloud. I don't want that nor do I want to be around it, good enough. Fenyman I am not. But then I don't think he could target shoot as good as I can.