I would go minimum 16x. 18, 20, or even 24 power - well, don't limit yourself or that great caliber! for the 600yd'er by not putting enough magnification "potential" behind it. You can always dial down, but if you don't have it... you can't dial up. Get yourself a pretty fine cross-hair too. Small targets at 300+ yards behind a really heavy/thick reticle can definitely be harder to work with. I put a Weaver V24 with "XDOT" - a mix of varmint and target dot I guess I'd say -- on my 22-250. Wouldn't recommend this particular scope, but that thin reticle and the 24 power magnification is great whether in the field or on the bench doing load development work. I like your idea though about keeping it to something that can go as low as 4 or 5 power. There are some 16's out there that might even let you get to 3 or 3.5x. I was considering a Bushnell 6500 3.5-16x50mm earlier for predator hunting but then I saw some not too favorable press about the 6500's in general. Take your time.