If you are using the Stadimeter to calculate speed you need to take two readings far enough apart to give a good solution. The TDC will calculate this info based on the gap between the two readings, only using one will not work.
You can also calculate from the Nav Map by marking target position, wait 3 minutes (3.25 if in metric) and make a second mark. Using the compass calculate the distance between points and divide by 100 to get speed. For example if the distance is 1000 yards the speed is 10 kts. You can then enter this info maunally into the TDC.
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