Common Misconception: "8 inch per mile squared"
One thing you hear flat earthers say often is that the curvature of the earth is 8 inch per mile squared.
This is only a rough estimate for the first few miles though.
It is way better to use the Pythagoric Proof: http://imgur.com/a/vI4XF
Original autocad drawing made by M. Kahnke (07/02/15)