I agree with the others here and particularly "richhelton". A variable scope with an upper range of 16 or 20 should be more than enough. I have a burris 3 x 12 on a remington 700, .308 tactical, and I've never felt under-glassed. I do have a 6 x 20 on my .220 swift, but varmints can be pretty small targets.

As a practical matter I rarely go above 12 power. Things get too jumpy for me. Even from a bench, at 15 or 16 power, the cross hairs move all over the target, just from my heart beat. Although, I am sure many others here are better skilled than I am.