Since the stadimeter is cursed and a one pixel error can result in a hundred or two hundred yard difference in measured range, I always time the run between two points three minutes apart. The number of hundred yards you run in that time is the target speed in knots. A cursed stadimeter also means the estimate course and speed button is also cursed.
If you are using radar, this is a dead accurate number you can rely on. With radar, you have the additional advantage of not having to expose your periscope with the target close aboard to obtain the information. With metric, the number of hundred meters traveled between two positions 3 minutes fifteen seconds apart does the trick.
According to the Submarine Torpedo Fire Control Manual, all shots under a 30º gyro angle are considered straight fire and can absorb a lot of range error without missing your target. That doesn't mean you can input 750 yards for a target 3,000 yards away! But it does mean that ballpark ranges work just fine for lesser gyro angles.
In a trailing situation, I don't like the 180º AoB shot all all. Just a tiny rudder movement of the target and your torpedo misses. I like to set up for a shot that runs parallel to the target, takes a 90º turn and impacts broadside. That means range is critical! But it also means almost no possibility for the target to maneuver out of the way. The action report that begins the RFB training manual is a recounting of exactly that situation.
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