Have you ever tried an easier way of calculating speed? This is the way I do it, it takes 20-30 seconds and is plenty accurate. ID the ship, and note its length. Put the crosshair just ahead of the ship and leave it. Start the stopwatch as soon as the bow crosses the crosshair, and stop it again as the stern passes it. The ship has traveled its entire length in that amount of time. Divide length by time, and you get meters per second. Divide that by .514, and you have knots! Piece of cake, and as long as the target's AOB is 30-50 or more, it's extremely accurate.
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