Right, which is why I am coming round to having 3 visible markers (nearpoint, midpoint, endpoint) plus an implied fourth, namely the viewpoint itself. If any two visible markers, line up, it follows that the viewpoint must be on the same straight line with them, and if they are the same height, the viewpoint must be the same height. So you don’t have to measure the height of the viewpoint, only the three visible markers.
Caveat: I say that if two points ‘line up’ then they and the viewpoint are on the same straight line. Strictly speaking, it means that the light followed that line, not that the line is straight. We need a further assumption that the light travelled in a straight line. There might have been refraction, e.g., or Einsteinian relativity effects might have caused it.
And here is another puzzle I have. The Wallace experiment is consistent with a flat water surface, but concave refraction. I.e. if the light curved downwards from the end marker underneath the mid marker, then curved back up to the viewer, this would be consistent with the observation of the mid mark being higher than the end mark. But it should be the other way round with the Rowbotham experiment. If you are standing in the water and can see the surface of the water 6 miles away then either (1) the water is flat and there is no refraction, or (2) the water is convex and the refraction is convex.
So if flat surface is true, there is no refraction at the surface of the water, but there is concave refraction 13 feet higher up. If convex surface true, than little or no refraction higher up, but convex refraction at the water surface. Does that make sense? And what observations would distinguish the two predictions?