Reg, G4FGQ wrote
"It would be nice to have 4, admittedly crude numbers for - MF and HF in
densely populated city areas. MF and HF in rural areas ."

In the USA, the Federal Communications Commission defines three
different types of service area for medium wave broadcasting stations.
The primary service area is the area where groundwave from the station
is not subject to fading. The secondary service area is served by the
station`s skywave such that there is no objectionable groundwave
interference. Nevertheless, skywave signals are subject to fading. The
third service area lies between the primary and secondary service areas.
It is subject to interference and fading. It is called an intermittent
service area. Assignments are made in an attempt to keep primary and
secondary service areas clear of objectionable interference.

FCC defines local channels as those frequencies reserved for stations of
1 KW daytime, and 250 watts nighttime, or less. Primary service is
limited by interference.

Regional channels are reserved for stations of 5 KW or less. Directional
patterns are often used by new stations to avoid interference in service
areas of existing stations.

Clear channels no longer are the sole domain of some high power lonely
US broadcast station. Clear channels are now often shared with one or
more high powered stations, and perhaps also shared with lower powered
stations during daytime hours, in an attempt to make better use of the
frequency while the high powered stations still serve wide areas.

The FCC specifies the following minimum field intensities:

City business or factory area------10 to 50 mV/m

City residential area------------------2 to 10 mV/m

Rural--all areas during winter or northern areas during
summer----------------------0.1 to 0.5 mV/m

Southern areas during summer 0.25 to 1.0 mV/m

The FCC has published additional guidelines for signal strengths in
towns of certain populations, to overcome often encountered noise:

Up to 2500 inhabitants----------0.5 mV/m

2500 to 10,000 inhabitants---2.0 mV/m

The needed signal intensity depends on interfering signal levels and all
other noise.

At higher frequencies, groundwave propagation goes away but noise
declines too. The FCC field intensities needed for satisfaction at
mediumwave broadcasting may be more than required for amateur
communications.

The information above is provided by John E. Cunningham in "The Complete
Broadcast Handbook".

The answer to Reg`s questions are incomplete, but I hope they help.

Best regards, Richard Harrison, KB5WZI