I believe the generally accepted method for neck turning brass for a factory chambered rifle is to turn it so that approximately 75% of the neck is turned. That is, about 25% of the neck is not touched at all by the neck turner. I say "generally accepted" because in all my research on this, the 75/25 ratio seemed to be mentioned in practically everything authoritative I was able to find on this subject. If you turn the neck down too much and shoot the brass in a loose factory chamber you risk the neck stretching too much and prematurely splitting.

Personally, unless you're shooting competitive benchrest, I don't see much benefit in neck turning brass. I say that having done it myself. Iown a K & N neck turner, which does a great job BTW. What makes more sense to me is to measure the neck thickness of your brass and sort it intobatches so that the necks of a particular batch differ no more than .001" of each other. Honestly though, I don't even like to do that anymore either, so I just fork over the extra money for Lapua (and here lately Nosler) brass which in my experience tends to be quite uniform from one piece to the next.