As I have said in other threads, the Eratosthenes experiment is flawed. You need at least 3 shadows to remove assumptions - preferably more to remove experimental error. The equipment must be calibrated precisely (length, everything being perpendicular, earth below it being flat, same time for the experiments, rotated around itself and between observers to eliminate equipment bias, etc.) Until this experiment is redone properly, it proves nothing. You get "the correct answer" depending on if you assume the Earth is round, flat or concave - a non-answer.
There are several threads discussing what you can actually see on/over the horizon. The jury is out on that one.
The 3 x 90° is theoretically true. The smaller the "triangle" to measure the sides and angles more precisely, the more it looks like a normal triangle and the more the terrain/ocean defects (hills, slope, waves, etc.) mess up the REAL measures. You won't be able to prove a round earth from that. Reality sucks.
My flight model directly proves this but discussing it derails the thread.
My 12/18/2015 cruise boat trip from Ushuaia, AR to S. Georgia Island taking 3 days directly proves RET, but I am sure it will derail the thread as well.