"Busy fitting a new barrel to my 308"
Then you already have the new barrel?
There are only a few reasons to neck turn. If you get a tight necked chamber, then you turn to get .004 (I think) clearance on the chamber for minimal working of the case neck. Shoot, bump the neck back down with a neck die, seat a bullet and shoot again.
If your chamber isn't small enough that you have to turn to make the brass fit, then you're where I am.
I turn for the obvious reason of consistant neck tension, and to give uniform tension when using a bushing style neck die. Forgive me if this is elementary, but a bushing die pushes the imperfections of your case neck to the inside, if you don't turn then your tensions will be way further off than if you'd simply used an expander ball style of die. Turned and using a bushing die is more even than not turning and using an expander ball, but probably the same as if you turned and use and expander ball.
Why turn then? For one, less work on the brass. You're taking it from chamber diameter to .003 smaller than the bullet diameter you're shooting rather than to .010 and then expanding it back out to .005 smaller with the ball.
Two is that Redding Competition dies utalize the bushings and are outstanding dies. With their setup, I get no discernable neck runout and only about .002" bullet runout.
How much to trim off?
What I was told and therefore have always done, is to leave the brass as thick as possible with no low spots on 90% of the brass you turn. For my .243 I seem to recall that left me with .014 wall thickness, but don't hold me to that. From there I figured the wall thickness, bullet diameter, neck tension of .003" and ordered 3 bushings, the one I thought I'd need and the ones to either side of it, for my Redding die. It seems to have worked out
Hope all that helps.