So this opens another possibility. I started this thread based on the idea that the sensor was a calibrated ref point and I was the subjective variation. How do we find out what the true (reasonably) iso scale is for a sensor. Gray scale card and external light meter compared to profiled monitor? This is a bit "Alice in Wonderland" but interesting.
There isn't a lot out there to help people see what the camera is capturing; at least not with the typical software. There are only 3 programs I am aware of that can show you the actual RAW capture; IRIS, ImagesPlus, and DCRAW with the right command line arguments. Trying to surmise RAW exposure from the RAW converters of choice is really futile.
If you're dealing with matte subject in consistent lighting, you can generally expose a grey card for +1 EC, use those manual settings, and expose without clipping. With my Canon 10D, I could meter for incident light with my Sekonic L-558 at ISO 32, and shoot the Gretag-Macbeth color checker with the meter settings with the camera set to ISO 100, and the white square would not clip in the RAW data. I believe metering for ISO 25 just barely clipped the green channel.
It's scary when you consider that people could take important shots, going through all the trouble of accurately metering for ISO 100, and getting 3x the shadow noise possible metering for ISO 32. Even ISO 200 metered for ISO 64 would have much less shadow noise.
A camera, set to an ISO, basically has two different ISO sensitivities; the ISO of average metered value, and the ISO of maximum unclipped white highlights.