Again, you are confusing the meter reading, which would give an f/stop and shutter speed for a given ISO. How the sensor would respond to this exposure is a different matter. According to the REI (recommended exposure index) that Japanese manufactures use, they may assign whatever sensor ISO they want, and the resulting sensor saturation may or may not be 12.7%. In your earlier posts you made some good points, but more recently you have gotten off track. Please read up on these topics before posting again. Doug Kerr has a good paper on the subject.
To determine the sensor saturation, one has to look directly at the raw file with a program such as Rawdigger. The first step is to determine the saturation value, which may not be 2^14 -1 (16383) for a 14 bit ADC. One then determines the raw value for the exposure (the ISO mentions an 18% card, but any uniform target (gray or white) will do as you mentioned. One may have to subtract an offset (Canon cameras). The quotient of these values is the percent saturation. The rendered sRGB value can not be used since a tone curve may have been applied.
DXO describes how they determine sensor sensitivity and how the manufacturer may deviate from the saturation standard. Look at the ISO measurement for the Phase One IQ180.
I'm not confusing anything. You're the one who's confusing metering with response and processing. You're doing this because you can only think it terms of ETTR. But that's not how cameras are designed to be used. Camera makers never intended photographers to base exposure from the un-demosaic values, as they provide no tools to access those values.
The meter doesn't produce an f/stop and shutter speed. The meter combines a luminance reading with the ISO speed (the speed you select, not the REI) to produce an EV. That EV is then biased by any EC that the photographer has applied. Then the biased EV is passed to the camera's exposure program. For example, if you're in Program mode then the EV is looked up on a chart (also considering focal length on some cameras) and then the camera sets the aperture and shutter listed for that EV. That program-mode chart should be in the back of your camera manual.
Metering works exactly the same way in a digital camera as it does in a film camera, as it does across digital cameras. My Canon compact, spot-metering my gray card, gives me the same exact exposure settings as my Nikon D90. This is all defined by ISO 2721. Metering has nothing to do with REI.
Once the image is captured, however, the camera maker has complete control over how the signals get processed. This is where REI comes in. REI is the equivalent of pushing/pulling film...the film has a rated speed (the ISO speed) but you can process it as if it were any other speed, depending on how you decided to expose the film. REI is the same thing. The manufacturers process the signals to produce, supposedly, a particular result...that result being an RGB image in the sRGB colorspace. Although not explicitely defined for REI (it is for SOS,) the sRGB image should have 18% gray right at RGB 118 (referred to as the "standard level".)
By extension, that puts 12.7% gray at RGB 100. As it is the expressed purpose of REI to match meters (see CPIA DC-004 Explanation) it is reasonable to expect that a meter with a K of 12.7 metering a 12.7% gray reference will produce an RGB image with values of 100. The problem is accessing that image, as all cameras apply additional processing based on defaults, picture controls, HDR settings, etc. So how to we see the base RGB image? We use a RAW converter that allows us to demosaic the RAW and apply white balance and nothing else.http://www.cipa.jp/english/hyoujunka/kikaku/pdf/DC-004_EN.pdf
Now...as to whether sRGB 100 actually represents 12.7% saturation of the sensor...that's a different topic. sRGB 255 may not represent full saturation of the sensor, but it's close. And again, actual full saturation is a concern of ETTR practitioners only.
And yes, I've read DxO and many of Doug Kerr's papers...including the one you reference, as well as the one where his exposure test produced RGB 100, 100, 100.http://dougkerr.net/pumpkin/articles/Scene_Reflectance.pdf
I see you edited your post and removed your erroneous comment about 18%. I'm glad you corrected your error, but I suggest that next time you read up on these topics before posting.