Flat earth people incorrectly apply the 8 in per mile formula. That formula is applied to the level line that is tangent to the observation point. Here is a utility that will show how much is hidden by the curve.
http://dizzib.github.io/earth/curve-calc/?d0=1429&h0=1314720&unit=imperial
No, that is the calculations he used. And he is right that the results you get does not match observations at every place on Earth. It is only right on average.
See explanation in my comment.