This is for SH3, but the same basic approach works with SH5:
http://www.subsim.com/radioroom/show...&postcount=461
best to play with map updates on, until you understand the concept.
as to "when" to fire. Ideally, you want the torps to hit at a 90 degree angle. With map updates on, you will see where your torps are heading to hit at 90 degree. With a slow moving freighter, you usually have to lead by about 10 degrees, which means firing at 350 or 010 rather than 000.