I would also recommend to have a sensitivity that suits you and keeping it consistent. You want something that allows you to move you crosshair with a persons head as he run away slowly, but not so slow that you can't turn around if someone flanks you. Try not to change your sensitivity at all if possible to keep aim as consistent as possible because it helps majorly when flicking the mouse to an enemy. If you are to fast you have less of a chance to be accurate and over flick the enemy.