With a 1:9 twist, you might be able to get away using 77 gr projectiles, but they probably won't be optimal. That twist is probably best suited to projectiles up to the 68/69 gr range. Using those, you should be able to reach out to 400-600 yd (depending on the wind and you) without too much trouble. You really won't need any cant in your base (ie. "0") to achieve those distances. As far as scope magnification, a lot depends on you. You could easily get away with 10X-15X upper mag limit for those ranges as long as you're comfortable with the sight picture. However, going with something in the 20X-25X range isn't going to hurt you either. If it's too much, you can always dial down a bit. One reason for getting the extra mag is that it might be useful if you ever decide to put the scope on a different rifle that can reach out a little farther. Buy once, cry once as the saying goes.