HV diagrams are determined by flight test after some empirical analysis. An interesting read is the applicable portions of the FAA Advisory Circulars 27 and 29 which tell one how to get the data. Also, the military has somewhat different criteria. One of the things that jumps out is that its determination is highly dependent on the test pilot's method/talent/experience. If you have the manuals available, compare the HV diagrams published in the Bell 205A1 RFM with the one in the Bell UH-1H. The aircraft are physically the same and you would expect the HV's to overlay each other pretty closely. They don't.
I have a personal problem with HV diagrams anyway. It is a sterile test done over a smooth flat runway. How many times do you hover over a smooth flat surface, OGE. If you spend much time in helicopters in the real world, it's a rare occurance.