Larry, ya know, that's something I've often wondered myself- not only how manufacturers arrive at their loading specs but also why more of them don't do anything at all. This is a technique that does not require a test LP, or even a turntable, and is something any manufacturer could do.
One thing to be aware of though is that individual units, especially those that are hand-made, will differ from one example to another. My thoughts are that the manufacturer could bother to chart the critical damping on the same sheet of paper that the bandwidth is charted on, included with the cartridge itself when you buy it... sure make my like easier- people are asking us for cartridge loading values all the time!