It is indeed rather easy, I personally think you would benefit greatly from not making a spreadsheet, but doing loose estimates in your head each time - it doesn't hurt in any way, and only serves to back up your sense of practical maths.
Seeing as he knows at which direction the convoy is going, he can draw a line that makes it's course. Very nice line, you should always drag lines.
Since he knows the convoy's speed and what time it currently is, he knows for how many hours he would like the convoy to travel before the sub meets up with it. He can use the SHIII ruler to measure the distance ON the distance line, and this will be the spot most convenient to meet the convoy. Then you just adjust your own ship's direction until it's poking at the point.
But wait a m......
are the distances in SHIII in KILOMETERS?
