Reg, G4FGQ wrote:
"At the back of my mind I have a figure of 5 dB per reflection. Is this
the right ball park?"

It could be right for some reflections.

E.A. Laport was Chief Engineer of RCA International when he wrote "Radio
Antenna Engineering". On page 236 Fig. 3.17 shows the effect of ground
conductivity on maximum field strength from a horizontal dipole antenna
versus its height in vavelengths.

Optimum height would be about 0,50 wavelength to most concentrate energy
at a certain vertical angle, 30-degrees according to the RAF Signal
Manual quoted by Laport. 30-degrees might hop 1000 kilometers.

At the antenna earth reflection point, frequencies between 2 and 16 MHz
are reduced to 95% of their prereflection field strengths by ordinary
soil from a dipole at 1/2-wavelength height. A reduction to 70.7% of
prereflection strength would represent a 3 dB power loss. So no harm
done yet by the reflection from an antenna over good soil. Lower antenna
height and poorer soil would attenuate more.

The angle at which rhe signal strikes the earth in subsequent
reflections should be the same as the first reflection from the antenna.
Conductivity and dielectric constant at subsequent earth reflection
points are what they are.

Shortwave broadcasters use vertically stacked horizontal elements to
concentrate the vertical beam to avoid multipath interference. They also
prefer targets reached on the first reflection from the ionosphere..

Best regards, Richard Harrison, KB5WZI