Perhaps I should apologise for being a little flippant with my observations on this.
However, I do recall, during my time working for the Queen, stationed at RNAS Brawdy South Wales in a ground based crash waggon, clearly hearing the transmission " Lossie Tower, Rescue one do you read? over." Etc. From a Landrover. I cannot remember whether these were VHF or UHF sets that we used. (1964)??
So if the ionospheric influence is also a variable how goes the calculation?