How to attack a convoy at a 45 degree angle

[Tweety]

I came across a convoy with 2 T1 (G7A) torpedoes left in bow tubes and 1 left in stern. What I want to do is attack a ship 3rd column in and 2 rows in.
I'm thinking of attack at a 45 degree angle, so the torpedo will attack the ammunition ship without hitting any other ships.

http://i681.photobucket.com/albums/vv176/captblight/SH3Img17-1-2010_812.jpg
This shows the problems I am facing. The T1 has a
range: 5-7.5-12.5
speed: 44-40-30

I sure there is way of attacking at a 45 degree angle and using the speed of the torpedo at a certain distance with a lead angle and known speed of ship.

Is there anyone here that would know the steps on how to do this procedure. I want to learn to do better manual targeting with map contacts on before I take the plunge and turn map contacts off.

I'm sorry if this has been posted before. I searched and didn't find what i was looking for.

I appreciate any help or suggestions.

Tweety