I haven't got SH5, but is there a rev/speed chart available here.
This method was used by both sides, and it's a simple matter of ship(engine) ID and prop rev count = approx speed. A 3 torp spread is usually more than enough.
I've nailed a ship at about 3-4K in visibility conditions of 50m, rain sleet and snow. This was definitely the luckiest shot.. but a good amount of calcs and obervations went into the aiming.