The exact solution for length of tangent (assuming station at GL) is:
sqrt(2rh+h^2)
where r is the radius of the earth
and h is the height of the observer.
To a first order the h^2 term is negligible.
So length of tangent = sqrt(2r) * sqrt(height)
In nautical miles the coefficient works out at 1.0634
In statute miles it is 1.2245
So 1 is an easy conservative value for a rule of thumb.
