Please re-read the quote I lifted from the book, posted at 12:22 yesterday.
It is clear that the Wahoo
waited as the target passed 90-port to obtain the 120-degree track that Morton felt gave the enemy the least chance to evade. If it were a 60-degree angle as you surmised, they would shoot prior to getting the 90-port angle (as I typically do in SH4).
[EDIT: Removed some verbage to help clarify the thread.] I'll settle for a few links:
http://en.wikipedia.org/wiki/File:FiringGeometry.png (this from the TDC entry shows track angle, although it is listed as "theta subscript track").
http://www.history.navy.mil/library/online/ss-doc-4.htm (see 4107.i).
I think Nisgeis has hit it close to the mark, as that makes some sense. I may need to play around with it on paper and in the game to satisfy myself of how Morton/Wahoo were seeing the problem (and given their success I can't argue, although in the game it very well might be different...).