Amount of magnification needed is a very personal thing. Just go to Camp Perry some time during the National's, you'll see guys shooting 1,000 yards with everything from iron sights to 40x scopes depending on class and match type. For a long time I advocated "the more magnification the better," but lately I've been rethinking that and slowly transitioning back to using less and less magnification and even shooting with iron sights much more often. There's a couple reasons for this:

1. More magnification not only increases the size of the target, but it also magnifies your movement and any imperfections in the lenses by the same amount.
2. The whole "Aim small, miss small" philosophy. The more you magnify the target the larger it gets in your view and the less you have to mentally focus on a good steady hold.
3. Learning to shoot well with little to no magnification will make you a better and more consistent shooter when you have the luxury of more magnification.
4. When you can't see your bullet holes or splashes on the target you tend not to over think or over correct after each shot.