Calibrated S-meters, don't exist. There is no such thing.
Each radio manufacture selects a signal level to calibrate the meter.
ICOM, selected the level of 50uV at 14mhz as equal to S-9. There is NO industry standard. However, it is generally accepted that each S meter division is roughly equal to 6dB.
Outside of these, it's all relative, dependent on a large number of installation variables.
My first receiver was a Command Set RX (BC-454?), no S-Meter. I give RST reports based on what I hear, not what any meter says - you can tell how loud somebody is without looking. No calibration needed ;-)
The only uses of an S-meter that I can see is:
1. If a guy wants to know the difference between him being barefoot and throwing his amp on. Or turning his beam. And even those are dependent on propagation and QSB.
2. If I'm trying to track down the source of some household generated noise.
Other than that, who cares about calibration. Every mfr is different. Try contesting...ever get a report less than 599?
73, Bob K7JQ