So the way to measure is clear but when the transom isn't in front of you to measure???
I think a lot of folk who do measure, measure the actual transom and not the vertical. But transoms are supposed to be pretty standard re lengths now. So if the vertical is 20" and therefore aimed at a long shaft how long would the actual be? The angle of the transom would have an impact here, but that seems to be fairly standard at 15 degrees I think
If you don't like my opinion, I have others.