First off, how you go about determining how much to trim is exactly opposite for factory and tight neck guns.
IF it is factory chamber you need to know what is the actual fired case diameter at the neck unless you have a cerrosafe chamber cast to measure. Many factory necks are .008-.010 over loaded round size with unturned necks already. Guess what, neck turning .001 off is not going to help you diddly squat with that large a neck. Total waste of time. If you are only .003-.004 it is really questionable then.
Factory 30 cal with an actual .348 neck, which is on the small side anyway .348 minus .308 gives you . 040 or .020 per side. Very few cases are over .016 at most. That gives you .008 total clearance. Now unless your cases are showing over .003 variance from side to side, IMO total waste of time to neck turn .002 off to make even larger neck variance of .010-.012 in a factory chamber. Think of it as putting a nascar engine in a Yugo. That is not going to work like you want and for all practical purposes impossible to measure an realistic difference in a factory size chamber and barrel. Answer, buy better brass RWS, Lapua etc.
And you are going to need at least .003 neck tension on a loaded round for magazine hunting rifle as minimum or the bullet can and will move under recoil in the neck.
This also means you will often have to go to a standard bushing die or send your non bushing die to Jim Carstenson at JLC precision in Iowa (www.6mmbr.com
and under "Tools") for him to convert it to a neck bushing. Cost is about $45 and week turn around. You have to be able to get the neck tension and standard dies often will not give it to you after neck turning. You do not need a custom die, just a bushing die with the correct bushing to give you the proper neck tension.
Now some will state cleaning up factory cases .002 gives more uniform neck tension (in a sloppy chamber). Well maybe, but after 3-4 firings your uniform neck tension has gone to pot and is in actuality all over the map unless you anneal anyway and still in a sloppy chamber. Hell of lot of work for nothing. Been there done that. Wanna try it, get a set of pin gauges and see which ones fit in what necks after resizing that 4th time. the actual variance WILL be .0005 to .002 on the neck.
Now if it is a tight neck gun, you cannot safely fire a case without neck turning it first so no way normally to measure an actual fired case on a new tightneck gun. Even if you could, it is a totally meaningless measurement anyway. Fired cases will expand to chamber neck diameter and then shrink some upon firing. You in reality want to know what is the actual "loaded" bullet/case diameter that will fit safely in that neck.
EXAMPLE: 300 WSM
with minimal tight neck of .338 with brass of .015-.017 and .308 bullet equals a loaded round of at least .338 to .342 with zero clearance. Not a good situation and it routinely will not even chamber. .338 is not even real tight for a 30 cal either. They are often .334.
In a tight neck gun, you are actually guided by the neck diameter of the chamber, not the fired case. Take the reamer neck diameter say .338 neck for a 30 cal, subtract .308 from it to get .030 or .015 per side with zero clearance. You need a min of .001 per side so that gives you a final turned neck of .014 thickness maximum.
You will not be able to take off over .002-.003 per cut and do it cleanly. I always set first cut to leave about .0015 per side. Trim all cases with the intial cut first.
Adjust your cutter for the final cut and use either power screwdriver or power neck turner at 120 to no more than 180 rpm. final cut is slow and clean. put the cutter each time on bag of ice or in bowl of cool water/alcohol to cool it between cases. The reason for that is your cutter head and mandrell will heat up and expand as you turn and that expansion will give you as much as .0005 or more variance if you do not cool it. Been there and done that.
Word of caution, these measurement numbers are ballparks to work from. For example a 30 cal bullet has a pressure ring and often pushes the case out past what the nubers add up to such as .0015 each side and .308 should be .338 loaded round but it might come up to .3085 or more, hence the minimum clearance of .001-.0015 per side.
Do you really want to lose the buck or elk of a lifetime to a bullet/case jamming with one little piece of dirt on a hunt? Seen it do it and the barrel actually had to be taken off to get the bolt out an only one little piece of dirt.
It is also a very wise practice to do only one case, load it, measure and see what you really have before doing a 100-200 cases.
Wanna guess how I know that one?
Old rule measure twice and cut once definitely applies. Go back to previous statement about doing one case.