Okay, I'm not sure where a question about a .308 has introduced the possibility of a "magnum"...

If you're shooting a .308, it depends upon your intended use. If the heaviest bullet you intend to send downrange is 150 grains, a 1:12 would be fine. If you're sending something 175-178 grains and heavier, you need to lean toward 1:10. I know a lot of guys who are shooting .308s with 1:11 (and fractions) as a compromise, but I don't see that they work quite as well as a 1:12 with light bullets, or a 1:10 with heavy bullets.