The best write up and data I have seen on this subject was the "team vertical" report on test done in Jamaica back about 10 or more years ago.
As I recall, the vertical signal strength to low angle DX went up "dramatically" within 2 or less wavelengths of the edge of the high water mark and maybe leveled off as "fantastic" from within 0.5 wavelength. But further and further away past 2 wavelengths, the signal strengths dropped away and had very diminishing effects. I don't recall how far back before the benefits were disappointing but that article has the answers you need. Just scale it for 160 or 80M vs their 40 - 10M data. By the way, I used a vertical as 9M6/N1UR at Layang Layang island in the Spratlys in 1998. 40 and 30M performance was "amazing" but 20 - 10 was good but not great. The vertical was placed about 100 feet from the edge of the water. So it would have been just under a wavelength on 40, just over on 30, and 2 - 3 wavelengths on 20 - 10. Ed N1UR _________________ Topband Reflector Archives - http://www.contesting.com/_topband