Here is the derivation for the dip of the horizon for flat earth. If this is the incorrect map, please send the correct flat earth map and I will make on for your map. If this derivation is incorrect, please send the corrected one and I will publish it instead.

Test it:

From 11,074 meters / 6.8 miles, the measured dip is 3.3°. Significantly more than flat earth’s predicted 0.0417°.