The gyro angle is simply the bearing to which the torpedo needs to turn to get to the impact point, based on the speed you set. If this (gyro) is 0 then the torpedo moves straight ahead out of the tube. In this case it doesn't matter what range you entered into the TDC dial. (the range is a factor to correct the angle to the impact point from the forward placement of the torpedo turning point, with reference to the angle from the periscope location) The range is only important if the torpedo needs to turn a significant amount of degrees(>15 or so) from 0. So, yeah, this is probably best to minimize aiming errors. One less variable to worry about. Set the periscope such that the gyro angle is 0 (or 180 if shooting from aft), then wait to fire when the juicy part passes the wire.
However, this all assumes the target speed is set correctly. Speed is the critical ingredient in a correct hit. It defines how much the torpedo needs to turn away (lead) from the direction of the periscope. I hope you meant you averaged the speed over 3 times 3m15s marks, so total motion over 9m45s divided by 3. Then 1 knot for every 100 meters, and don't ignore the decimal part that is left.
AOB needs to be set to whatever it will show when the wire is on it. If you setup your position and subs heading perpendicular to the target's track, then when the target is dead ahead, his AOB will be 90. This is when you want the torpedo to hit because it shows it's broadside to the torpedo. With the periscope temporarily on 0 degree you set the AOB dial to 90 left or right. Once the TDC switch is set back to auto-update (AOB will be automatically adjusted when you turn the periscope), you can adjust the periscope to get the gyro back to 0. You'll see that the persicope is now around 5-15 degrees from 0. This amount of lead is roughly proportional to the target speed. And now it is just patience. And don't forget to open those doors.
If you decide to shoot with a salvo spread, then you need to be sure you know how big the target is at that range (in terms of degrees) At 1000m 1 degree is 17.5 meters wide. So take the target's length into account. If you don't know and just venture a guess then the 2 outside torpedos of a 3 spread might pass harmlessly aside. Waste not, want not.
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