I too, found this AOB calculation frustrating at first - even though I was familiar with the concept from a previous subsim game.
3/4 of the way into my first patrol I decided to try Wazoo's 90-degree angle on the bow method, and I started to have much better success.
The method has as a premise that you are able to get your ship lined up for a 90-degree angle shot, but then it lets the TDC make
minute AOB adjustments for you automatically if you shoot slightly to the left or right of what's ideal. (It also gets you started on manually entering information into the TDC, which I think is a lot more realistic than the 'point and shoot' method.)
You can find Wazoo's tutorial here:
http://www.paulwasserman.net/SHIII/#...ols%20Required
but it might give you a bit more information than you were hoping for.
You can also try jiggering with the bearing setting on the TDC for more of an 'off the cuff' situation. That has helped me on more than a few occasions. (Sunk my fist destroyer that way.)
Hope this is of some help.