More Flexible Speed Rule.

Using the 'mark chart, wait 3 minutes and 15 seconds, re-mark chart and measure distance' rule, if you don't have the time to wait for a full "3 minutes and 15 seconds" to determine the speed of a target, you can measure the distance the target runs over any time period and then use the formula:

Distance (in meters) x 2 / Time (in seconds) = knots

This is easy to remember but has a slight error in it. It overstates the speed by 2% but will give you the speed within a quarter of a knot for speeds under 20 knots. If you want to be really accurate, the 'x 2' should be 'x 1.969'.

Obviously the longer period you time for, the more accurate the answer will be.