3.15 rule is quite simple. Make a mark on the map where your contact is and start timing. When 3 min and 15 sec has gone, make a new mark on the contact. Now take the ruler and measure the distance the target has travelled, and the speed will be 1knt for each 100 meter.
So, if you measure 1.100m between the two marks, the target speed is 11knt.
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