I wrote:
"Another (option) would be to find some curves which have already been
constructed."

Found some curves in Pete Saveskie`s "Radio Propagation Handbook" on
pages 15, 17, and 19. They are from CCIR and may be available from them.
They are for field strengths over sea water, good earth, and poor earth
at 16 frequencies from 100 KHz to 10 MHZ.

Best regards, Richard Harrison, KB5WZI