Originally Posted by Mark 1
The exact solution for length of tangent (assuming station at GL)...
In nautical miles the coefficient works out at 1.0634
Your exact solution assumes that the ray travels in a straight line. In fact it is curved by refraction in the atmosphere. The magnitude of the effect depends on the frequency: for light it's about 10% greater distance to the horizon than the geometrical solution, for VHF it could be rather more.