Seeing something with the eye is no proof. I have seen Godzilla attack Tokyo countless times, still I am quite sure it never happened. The earth might look round but it still might be an optical illusion.
Not sure what I am supposed to do now that I tapped a globe to my forehead with my Fingers pointing at Johannesburg and Sydney, but as far as I heard most world maps and globes are highly inaccurate.
When I think the Earth might be flat. It is more the question if we are not just curved in other dimensions but actually flat in the ones we know. Something along the lines of the earth is flat but its ends are completely connected in the 5th dimension which makes it appear round in our eyes.
Maybe you should just let people believe the flat-earth theory. They don't really need to know any better.