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Scott, what I was driving at was the 5dB NM for 50 is probably a bit optimistic for two reasons. First, all propagation paths have variability. Line-of-sight paths have the least, so the received signal strength doesn't change much moment to moment. When a VHF or UHF signal has to be bent over an obstruction, the actual scattering mechanism is less stable (air density, terrain roughness will now have a greater impact.) Thus the received signal strength will vary more moment to moment and day to day. Add another scatter point in the path and there will be even more variability.
Propagation predictions use statistics to account for this behavior, and there is a statistic attached to the NM prediction. I don't know if it is 50% or 90%, but there will be chunks of time where the predicted signal strength will be far less than what a 5 NM implies. To account for this, we normally recommend a design target of 10 to 15 NM for LOS and 1-edge and 15 to 20 NM for 2-edge paths to account for the variability.
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