What may start out as a "subjective" measuring process (at least in that it involves a subject doing the measuring, not an object) can swiftly become quite "objective" when it gets confirmed by another "yardstick". In the case of water dowsing this is, of course, the well-driller's drill-bit. In the case of energy dowsing we may find that some electromagnetic measuring device (EMF meter, radio, spectrum analyzer, etc.) confirms our initial findings. That's what I'm working toward. I haven't used frequency to assess earth energies when in the field as the process seems pretty cumbersome compared to what I normally do.
Regarding other scales/systems, I use the von Pohl scale, modified for both positive and negative values, to get an admittedly somewhat arbitrary fix on what I'm dealing with. I use it because it's a way of communicating with other dowsers who came into the process having read von Pohl's research on cancer clusters and the values he found that were likely to cause problems. Comparing those with other geopathic energies at least gives me a work sequence when I'm diverting/dissipating energies at a home. But I've had to move beyond it to get more specific as to which energy "layers" are affected in a human or other animals. A specific, quantified effect on the "physical" layer is more harmful than one on the "etheric", and that's in turn more harmful than one on the "astral" (unless you're O.K. with really nightmarish dreams!), etc.
"Develop an infallible technique, then place yourself at the mercy of inspiration." (from a craftsman, but it applies widely)