I have brought a topic I asked about from the Q&A section to this place.
https://www.theflatearthsociety.org/forum/index.php?topic=92672.0
In his reply, wise talks about 'calculations'.
Where can I find those calculations?
Who did them first, using what kind of raw data?
He talks about 'studies'.
Where can I find those studies?
No idea who did them first and what exactly they are calculating, but a common one is the observation that on the equinox the sun is directly overhead for the equator while for someone at 45 degrees north, it is at 45 degrees.
Under the assumption Earth is flat, that gives us a right angle, isosceles triangle, with the common sides being 5000 km.
That places the sun at an altitude of 5000 km.