Just what I do.
If someone has a better method. Then please disregrd this post.
On the PT, jsut like a tank, I use my mouse as a cursor. ( also in SHip to Ship fighting) I place my mouse as the crosshair on one of the gun sights.
Then jump to the rocket position. Make sure your mouse will not interfere with anymoving, because now when you go to the rocket position, your mouse is the cursor.
Now the fun part. Find out how long/far your target is. Add Kentucky windage, and fire away.
I usually can hit the SHips, but as far as other PT boats. Forget it.
Practice is the only thing that helps., I estimate at 1k away Im about 3/4 inch in hight above, and aiming slightly in front of moving target. I dont have range of rockets, so I would love someone if they can give abetter description.
Cheers