I have a few problems with this. Firstly, I should not expect a laser to maintain phase coherence much beyond 5km at sea level because of atmospheric turbulence, so the beam may diverge by an amount comparable to the miniscule angle which is being measure. Secondly, the refractive index of the optical path is not known, but the fact that the surface is cold, possibly colder than the air above it could well lead to a temperature inversion curving the sight line downwards. Water has a considerable effect on the refractive index, so when trying to measure such a miniscule effect, ignoring the optical properties of the air is the height of stupidity. Even if the surface of the lake followed the curvature of the Earth the difference in surface height over 15 miles would be about 5cm. But the surface is unlikely to be perfectly spherical as it will be influenced by the local mass distribution (the mountains). Finally, when I last looked at Ex 20, I saw no commandment to believe the Earth is flat, but I did notice the 1st and 2nd commandments. This pre-occupation with a morally irrelevant issue verges on idolatory, and presents a bad witness to non believers. Please leave the Almighty out of the discussion. Whether the Earth is round or flat has no bearing on the Gospel message.