Mike Powell wrote:
"Our graph paper was drawn to 4/3 earth radius."

VHF and higher frequencies are generally said to require a line-of-sight
path.

Terman writes on page 820 of his 1955 edition:
"For the standard atmosphere, where dM/dh=0.036, the equivalent radius
is 4/3=1.33 times the natural radius of the earth.

There are times when the atmosphere doesn`t bend radio waves toward the
earth and even bends them away from the earth. If you need to build a
high reliability chain of microwave relay stations, you must provide
enough path ground clearance so that you`ll have paths when the radio
waves bend up as well as when the waves bend down. You pick the excess
you can afford for the statistical reliability this excess has been
shown to provide. I recall picking k=2/3 actual earth radius. This lead
to purchase of this type of graph paper to plot the paths on showing
land contours and possible obstructions.

Terman also om page 820, gives formula (22-7b) for:
Radio horizon in miles=aq rt (2h). where h is the height of the antenna
in feet. You can approximate many of these in your head.

Best regards, Richard Harrison, KB5WZI