Just a quick estimate:
Most of the time in SH3 you have a wind speed > 12m/s and most of the time you shoot torpedoes with a depth of < 15m. In this situation the failure probability is
0.25 + 0.75*0.5 = 62.5% failure rate in quite common situations
I really think the 0.75 is over-kill.
Regards, LGN1