This Round Earth Distance equation computes the maximum distance two people would be apart via line of sight so that one person could just barely see the tippy-top of the other person's head. If this calculation does not work, this might just prove the Flat Earth Theory. See how to test the theory below.
Find a locale near you that has what appears to be a level surface. Check topographical maps to identify a place where there are no elevation gains across a span of at least a couple miles from your start point. You may be able to find a stretch of highway that matches. If you live near a desert, even better.
Place a brightly colored object where you start whose bottom edge is the same height as your eye level. Make the top part of the sighting device an object like a big red bulls-eye sitting atop a shiny black structure. Both teh top part and teh suportting structure are going to need to be large enough to see with powerful beinoculars at a distance. It might look something like this. Be creative. But remember you're going to try to spot it at a pretty large distance
Carry a GPS to mark your begin and end locations (or use a tape measure if you don't trust a GPS). Or maybe you can borrow from somebody on a road crew or from the police department one of those wheeled devices that can you walk-off long distances . Or worst case you can drive and stop frequently to check your sighting of the stationary object which remains at the start point.
As the observer you'll need to carry a cell phone so you can report your distance from the start point when you can just barely see the tippy-top of the object .
You'll need to carry a powerful set of binoculars to get a good sighting of the object.
Continue traveling until you can no longer see the black base structure but can still see the red sighting marker atop. Check your distance and compare it to the value you get when you put your eyesight level into this vCalc equation for Round Earth Distance. If you can still see the black base structure when you have traveled more than twice the distance that vCalc estimates, you have proven the Earth is Flat.
Leave me comments if you prove the Earth is flat !
If you were over six foot and your eyesight level was about six foot, then entering 6 feet into this equation and choosing miles as the output units would give you very nearly 1.5 miles. That means that you should just loose sight of the base and still be able to fully see the bulls-eye atop at about 1.5 miles from your start.