If that equation shows the Earth is flat then why is it that we can't see for a long way away on the ground but we can when we're on a hill and even farther on a plane.
When you increase your altitude you are changing your perspective lines in relation to the earth, pushing back your vanishing point. The vanishing point, beyond which no man can see, is created when his perspective lines reach a certain angle smaller than the eye can see. If you increase your height you are changing your perspective lines and thus can see further before all sight is lost to the vanishing point.
The same effect is found on a 3D video game which assumes a flat surface. When you increase your altitude you can see farther because you are so much higher than everything else. Your computer's resolution is better able to see something below you than off on the horizon where the pixels are linearly squished.
When you increase your altitude on a plane you have pushed your vanishing point back.