[Nisgeis]

To answer the maths part, almost, but not exactly, because the target has width, so the angular length will never resolve precisely in that way, as long as the stern is rounded you will always get larger angular length than you would if you were just measuring from bow to stern along the keel. As you probably know the angular length hardly changes around AOB of 70 - 110, so as Vanjast says you will be better off eyeballing it from the posiion of the kinposts.

You could, if you really want to use the measurement between masts instead, as they are central, or one of the posts of the goalpost type masts (obviously the same one of each pair).