[chordatum]

I agree. That particular horizon figure is the only one I know off the top of my head, due to reading up on naval ballistics. Hence the remark about sea level.

Anything else requires some napkin math about the circumfrence of the earth and radial offsets for non-sea-level terrain, combine with a bit of triangulation, to estimate skyline impact. Then, heaped on top is the effects of local topography contours and relative relationships with other human structures.