The easiest way to get started with AoB measurements is to...
1) Leave map updates turned ON
2) Plot the target course
3) Position your sub on a 90° beam ahead of the target
From that position, AoB can be determined with two simple formulas based on your bearing to the target. As you become more adept, you'll be able to turn off the map updates...
The accuracy of the AoB measurement is dependent of course on the accuracy of your estimation of the targets true course. But if map updates are on, you can plot it without error. It's a good way to get started easily on the concepts.