A theory is only as good as the predictions it makes. For example, round earth theory accurately predicts the dates of eclipses, the times of sunrises and sunsets, the locations of stars and planets, the distance at which a ship will disappear over the horizon, etc.
Flat-earthers: please name any verifiable prediction based on flat earth theory, and show the math. For example, "the sun will set in Los Angeles tomorrow at 7:35pm, and here is the flat-earth based formula that calculates the time."