The red/green lines are neat, but not very useful. The most important thing to know about AA is the "golden angle" for the gun that you are using. This angle will hit most planes that are within range of your guns but not too close to your ship. The red lines can potentially help you hit planes that are either flying very low (torp bombers) or are very close to you. You need to line the green lines up with the shadow of the planes, and then try and put the red lines either on top or above the plane that you are shooting at. The only thing to do is practice till you start getting the feel for it.