Turn it up to 10 and use your left joy stick to strafe left and right to line up your shots. The sensitivity 10 is usually just for fast reactions with shotguns, fusion rifles, rocket launchers, and escaping tough situations. You can't escape fast enough if you're turning slower than a turtle. Use strikes to practice with that sensitivity. You can always turn it down when it feels too fast. Strafing is the key, however. That's how most good snipers earn their name in the Crucible.