Haven't played SH3 for a long time, changed from computer and did not install it yet.
I started with GWX and did the manual targeting from the beginning.
The Fast 90 method, where you position yourself on 90 degrees from the target is, as John said, a good start.
Range does not matter with this method. Even if you want to fire a torpedo sooner you can correct easily.
Have a look at this:
https://www.subsim.com/radioroom/sho...d.php?t=169935
Happy hunting, Marcel