As a tangent (?) to that thought, couldn’t he just go out onto a flat calm sea and ‘prove’ how flat the earth is, right there? Seems to me, if he wants to prove flatness, leaving the face of the Earth would not be the way to prove it; in other words, the higher he goes, the rounder it gets.
Bonneville Salt flats, the cars drop below the horizon.
About six feet at three miles.