Here is a handy chart to print out and take on experiments or show others. This tells you various distances an object would be below the horizon. IF YOU STILL SEE some object at this distance then “We have a problem Huston.!” By the way, the mathematicians gave us this figure based on the earth being 25,000 miles in circumference, using spherical trigonometry.
Print this out and take it with you when you talk about the flat earth.