Very good and interesting information Richard. Though I have never considered the readouts on my ultralights to achieve metrology grade accuracy. Rather simply a relative indication of signal strength.
It is good to know, however, that there is relationship to "real" field strength. Even though this will certainly vary unit to unit. I would think that throwing one of the baby FSL antennas or any external antenna would change the delta considerably.
I still prefer a good old analog "S" meter though. Especially when calibrated properly.