This is strictly a "for what it's worth" anecdote and you can take from it what you will.
I have a Forster case trimmer with a neck turning attachment I use for case reforming. It works fine but i really prefer neck reaming, especially when I'm needing to push the shoulder back. This causes that nasty thickening of the brass at the new shoulder that needs to be removed. A reamer does that well. For forming some 308 Norma I bought a Forster .308" neck reamer and it worked well in the Forster rig: it simply replaces the pilot. The reamer is sold for removing the thickening of the necks due to brass flow and is supposed to be used on unsized necks: A change in expander ball size facilitated the use in the Norma forming.
Anyhow.... I have been shooting a bunch of military 308 brass and wanted to take off .002" of neck to bring it down the spec of some commercial Winchester I had on hand. Instead of neck turning, or making a new expander ball, I tried progressively closing the neck with a Lee Collet die. My "datum' point was when the case neck became small enough for the reamer to just skim the carbon from the neck. At that point I turned the die in about a 20th of a turn, sized the case, then rotated it slightly and sized it again, then put it in the Forster trimmer. It took off about .001" as measured in several places around the circumference of the neck. I turned the die in the same amount and again sized the case. Reaming took off another .001 inches. I set the die and sized and reamed a handful of cases. They all came out with the same neck thickness, matching the Winchester.
I have no problem shooting the military brass as it comes from the plant but it was an interesting experiment.~Muir