Some people may wonder if this process is really effective. Because your Kate and the target ship is moving towards each other.
However, if your initial "mark" is correct(if you guessed your approach angle correctly in the first place), all you have to do is keep the target on your sight's angle marker.
If you keep the target on the numbered angle marker of the sight, as you and the target approach each other your "actual angle" will change and become something different than the initially guessed angle. However, because you are keeping the ship on the numbered mark, your heading also changes slowly with the movement of target ship. This means you keep on maintaining the correct lead amount for the torpedo drop, even if you and the target ship's relative angle keeps changing.
But this is purely theoretical, and most of the times, it works because ships are so large and slow, and you are much faster than them

The small difference in angles are almost negligable.