The actual "SilentHunter4 Manual.pdf" file found in the Support folder of the game has an OK series of instructions for targeting, as the game does it, and isn't a bad place to start reading. If you really want to do some in-depth reading (I know, I know, bad pun), then check-out the real
SUBMARINE TORPEDO FIRE CONTROL MANUAL on the
https://maritime.org/uss-pampanito/ site, one of several good sources of old information.