No flank speed when shooting - unless you're running from a persistent DD on the surface, and use the rear tubes...

Ahead 1/3. Can you see the ships' "shadows" in the fog / rain / wind? If you can see them, they most likely can see you, especially during daylight hours. Have you plotted their course on your NavMap yet? That would help you immensely in figuring out where they are going to be after a set time period. What you describe with the white "X" is where your torpedo is going to go, and where it is projected to impact what you set the PK on (Position Keeper). If you see it going off at a weird angle to your boat instead of straight out and then a pinch of a bend, say no more than 15°, then you should try to come straighter in on the target, being mindful of range. There are a few tutorials on targeting, whether auto- or manual- up in Rockin Robbin's
Sub Skipper's Bag of Tricks thread. Be sure and visit THEBERBSTER's thread
SH4 <> TUTORIALS <> HOW TO DO IT <> INFO <> DOWNLOAD LINKS -STICKY INDEX. Those two fellows point you at just about every useful tidbit of info there is to be found on SH4...