Proper Blind Exposure with a Gray Card

The thread has got a little messy and so please let me check what seems to be the thread of estimating exposure without reference to the histogram:

The theoretical argument really depends on the fact that digital responds more linearly than film and exposure generally needs to be adjusted downwards compared to film to avoid blowing highlights in a typical daylight scene.

Metering responds differently taking a reading of one colour channel only. Depending on the scene and the ambient light balance I assume. Since lighting more or less goes from red to blue then metering from red on an outdoor scheme will overexpose more than in incandescent lighting, no?

If the in-camera metering is an estimation of what seems to be going on in the scene then I think some manufacturers are doing very well in their estimates. But estimates are all they can be, just guessing a lot of photographs have strong daylight highlights and a green or gray foreground or face similar to 18% (or 12%) saturation. When you look on it as that simple then perhaps it isn't surprising that manufacturers generally get within half a stop of the ideal.