That's pretty handy info
what would be the maximum preferred? There about .010 thick right now I hope I didn't junk them!
I'm just a hack and don't turn my necks, but if that chamber/throat is a true .3390" in diameter, than you can live comfortably with a neck wall thickness of .0135"-.0140". Turning your necks down to .0100" seems a bit excessive, with regards to that particular throat measurement that you're throwing out, but maybe 300WM brass isn't typically .014"-.015" thick? It could be that you're starting off at only .0130", I just don't know where 300WM should be.
Usually, you'll want to turn to your necks to 'just clean up' the high spots and turn down, what...about 90% of the neck's length?
You don't want to thin them out too much, as that will weaken things a bit more, moving forward and you're spending a lot of time doing this, so you don't want to throw your cases away after only a couple of cycles.
Find out where your necks mic at before turning and use a precision ball mic at a few spots around a decent sample size of cases. Get an average 'thickness'. If you're above .01400", turn them down from there. If you're at .01350", turn them down a bit more just to clean them up and call it a day, since you're below where you need to be for that particular throat.
You're mainly focused on removing high spots around the circumference, unless you have to thin them out to get them to fit, in which case, you probalby should have gone with a wider throat IMO.
Chris