You might want to try this "
AoB Test Mission" I have on the
Optical Targeting Correction mod thread. You'll have four different merchants approaching from different angles, and separated by a small amount of time. The Porpoise sub is stationary, and at periscope depth. The mission was designed to help with learning manual targeting, specifically counting the periscope Telemeter divisions to determine range and AoB with the aid of the range Omnimeter. Since you're using OTC, the training mission may be of use.
If they scatter as you plunk the first one, you'll have to get on your horse and go after them. Their mission routes aren't very long, so don't be surprised if they disappear on you after a short chase. The mission is JSGME compatible, so you just need to add the mission to the "MODS" folder and activate it like any other mod.