How tall is Yertle the Turtle?

That ego-maniacal Yertle, proclaiming himself king of all that he sees, calls for a stack of 500 turtles to sit upon.
And now Yertle the Turtle was perched up so high,
he could see forty miles from his throne in the sky

So tell me: how tall is each turtle?  Assume the earth is a sphere with radius 4000 mi, that Yertle and the reset of the turtles are a uniform height, and that Yertle's eyes are on the top of his head.  The first correct solution (posted as a comment) will earn a cool drawing of the solver on top of Yertle.