[CapnScurvy]

Your right Hitman, in real life target sightings, the AoB was determined by pure experitise. The well trained observer could do well in sighting direction of travel as well as range to target with just binoculars.

In the game though, the 2D effect of looking at a flat monitor, that is simulating a 3D image, causes one to lose the ability to preceive depth. It's like losing sight in one eye, you can still see images, but depth is difficult to master. This makes in-game sightings by visual observation difficult as well.

But, as you know, if you want to set the game for maximum realism, checkmark ON "No map contact update", "No weapon officer assistance" in the Gameplay Settings and make your own sightings and notations on the Nav Map.

For you newbies, to use the AoB Calculator with the maxium gameplay difficulity:
(It would be best if your sub is stopped or at least on a steady course)

First, find your sub's course heading (found at the TOP of the Compass in the Quick Controls Panel of the game) and turn the middle Compass Dial of the Calculator to match the black triangle of the outer (Blue) ring.

Second, site the target ship with the periscope; TBT; or binoculars. Take the view-to-target bearing (the degree numbers found along the top of the device), and find the same bearing along the outer most part (the Blue ring) of the Calculator. With this bearing number in front of you, turn the inner most Target AoB Dial to what you believe to be how the ship appears from your original sighting. The target ship's approximate course heading will be in front of the black arrow of the inner Target AoB Dial.