Upload & Sell: On
Sorry to say, but the whole "Mid Grey" requirement is incorrect. It is true that a reflective meter reading of a middle grey will yield a Sunny 16 value, while a dark or light won't.
The assertion that the Sunny 16 is wrong because of the reflective reading is backwards. The reflective reading is what is wrong, as the reflective metering is predicated upon the assumption that the subject has a mid grey tonal value that you are metering (hence gray cards).
A reflective meter reading of three different subjects (one black, one white, one gray) of solid tonal value will return different readings ... however, if the image is exposed for those values ... all three images will be gray, despite the fact that they range from black to white (again, based on the meter's calibration to the assumption that a mid gray tonal value is the reflectance of the illumination present).
I tested this out many times on slide film years ago. I could hand you three slides that looked indiscernable as to which one was the black, white or gray subject. The reason why ... a reflective meter is predicated on the assumption that it is supposed to be middle gray (average scene) ... which btw, green grass makes a rough approximation ... and the corresponding exposure will indeed yield such a middle gray value.
I met an old Chinese man in Trinidad once who would meter off his arm, and then make the adjustment from that since his arm never changed color. He could use it just like one would a gray card, again understanding the difference between how a reflective meter works vs. the light falling incident upon a given subject. Granted change your orientation away from the key lighting of the sun, and Sunny 16 no longer applies.
Now, conversely ... take the same three subjects and place an incident meter in front of them to measure the amount of light falling on them, and no matter if you are metering in front of the black, gray or white subject, the incident light meter will yield the same reading. This is because, the subjects are being illuminated by the same amount of light.
As to the Sunny 16 rule warranting adjustment for backlit situations ... well then the subject is NOT being illuminated by the sun. Instead, it is being illuminated by the open sky, which does in fact require a different exposure. Sunny 16 = Sun + Sky. Backlit is without the sun. Take away the sunlight and you now only have the skylight (think incident light meter) which is of course, not as much light as with the sun.
For "Fast f/5.6" the math works out that f/5.6 is three stops open from f/16 (+3 EV). A corresponding offset of -3 EV would then be warranted. 2^3 = 8, so a factor of 8X your SS would represent the three stop diff, i.e. 100, 200, 400, 800. In that regard, adding a "0" becomes 1,000 (10X), and the diff between 8X vs. 10X has a little wiggle room for a slightly less exposure, but is still in the ballpark.
Sunny 16 is not a panacea ... but knowing your EV values can allow you to assess the light that is falling upon your subject. Not every day is Sunny 16, but understanding the difference between incident light that is illuminating your subject (the gist of Sunny 16 in lieu of an incident meter) and metering how much is being reflected / absorbed by your subject can be key to knowing whether or not your reflective meter is risking to get fooled by things like strong backlighting or dominant tonal values (dark or light).
Studio's use incident meters for a reason ... they tell you how much light is falling onto your subject, no matter the tonal value of the subject. Heck, they'll even meter the light before the subject even comes into the room.
Reflectance of light is obviously part of how light gets from the source, then "bounces" off the subject to our lens. But, one doesn't need to compensate for the reflectance value of incident light, that is what generates the tonal (hue) variance to provide you with the tone / color that is then captured. You don't have to do it twice. Exposing for the incident light is sufficient. Exposing for the reflectance value ... and, yup you have to reverse compensate for the tonal value of what you are metering off of. Not so with incident light values.
Sunny 16 (and other EV values) just gives you the ability to carry around a mental incident light meter.