When making long-distance observations it’s important to use the correct math.

This web site uses the correct formula:

http://walter.bislins.ch/bloge/index.asp?page=advanced+earth+curvature+calculator

h = hidden height
s = distance between observer and target along the surface of the globe
h0 = eye height of the observer measured from the surface of the globe
R = 6371 * (1+k)
k = measured coefficient of refraction
If k is not measured:
0.33-1.0 or more over water, higher on clear days
0.13-0.17 over land with high elevation
0.17-0.33 over land at low elevation, overcast daytime
0.33-1.0 over land at low elevation, other conditions

For a larger selection of premeasured values for k under different conditions, see:
MCToon.net/refraction