Correct, you need range and AOB first. What’s happening is not what it seems - the game is calculating speed based on bearing change and not doing the “fixed-wire method” which times the transit of the vertical wire. This is clever of Ubisoft, as this was a method used as a backup (as was fixed-wire), albeit with a slide rule and not a magic notepad.
For that calculation, you need a range, an AOB, and time. 30 seconds to a minute should do it. And you need to keep the target locked in the scope as stork said.
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