It will be 0.35 of a mile, plus 3 ft.

How did you work this out? []

The circumference of the earth is 360x60=21,600 miles. That is because one mile equals 1 minute of a degree.

From that you can calculate the radius of the earth.

Then it is just a right angled triangle after that where one side is the radius of the earth, another side is 50 miles and you can calculate the length of the hypotenuse. The altitude is the difference between the hypotenuse and the radius of the earth.

A similar thing occurs when ships have to calculate at what distance they should be able to see a lighthouse.