Angle on the Bow is the killer question for all of us.
First of all, let’s make sure you understand what it means.
AoB is the angle from your target's heading to YOUR sub. If the Captain of your future doomed target were to be looking helplessly at your sub, the bearing he is looking is the angle of (his) bow. AoB is always the angle from the point of view of the target.
AoB is completely independent of the heading of your sub but is totally dependent on the heading of your doomed target.
The most common and crappiest way to determine AoB is to estimate it by the appearance of your target in either your periscope/TBT. Your recognition manual gives some angle views of the specific ship to help. This is quick but inaccurate.
If you have enough time you can plot the course line of your target on the map. Then it is a simple task to measure the angle from the course line to your sub.....but where do you measure along the course line?????
That’s the kicker. You have to measure from where the ship is on the projected course line. Any errors will result in large errors in AoB. Rats.
Target Zig-Zagging kills AoB calculations (that's why they do it)
There are manual AoB calculators that some of the smart guys here have made and have links for. These are very nice but some of them can be complicated. But then calculating AoB IS complicated.
Sorry I don’t have a more precise answer for you. This is one aspect of the game where your personal experience (in the game) makes you a killer Kaluen.
Good luck and good hunting