VB doesn’t use any device,
but if you timing 1200 ms rotation and when the ball takes ~1200ms for rotation you know that after that it will travel fixed amount of rotations. Fixed amount of rotations has it’s time to be completed, for example 10 sec.
Then 10s/rotor time will give you how much rotor will move in remaining time.
VB assumes the ball will drop at particular diamond so the drop point is constant.