The best way I found to practice your AoB estimates is to leave map contact updates on and look at the attack map (2nd button from left on Nav tab).
When your scope is up and you've just input all the goodies, check that screen and a white line will show where your estimated target should be and the line shows it's estimated heading. Go back and adjust the AoB until you're happy with the results.
After doing this for awhile you'll get a feel for estimating the AoB quite accurately the first time you try and then you won't even need to check the attack map, you'll be so damn good at it.
