Hi. The formulas used to calculate radio station field strengths are not a matter of "1 Unicorn + 1 Unicorn = 2 Unicorns." They are matters of real-world importance -- such as serving a community without interference, and emergency communications -- and measurable results. Wealth on the order of millions, and millions of lives, are on the line every day based on those formulas, and some of our best minds determined their validity to a high degree accuracy. I have taken probably thousands of field strength readings of broadcast stations, and I can assure you that the formulas, which include terms based on the curvature of the Earth's surface, are valid 100 per cent. of the time. Light waves behave similarly.
As far as many ancient people believing in a "flat Earth," so what? Most people have always believed in a grab bag of preposterous fictions, and there's little that can be done about that. When I say that _we_ have known about the Earth's sphericity since Eratosthenes' great experiment, "we" does not refer to the great unwashed, many of whom still believe in demons and witches, but to the those who make it their business to know, whose moral compass leads them to approach the truth as closely as possible, and who make high culture possible.