I don't have long, but did want to clarify a few things:
Yeah, I agree that RGB% measurements don't have much useful meaning unless they are made relative to the display's actual primary colors, as I said way back earlier in this thread.
In ColorFacts, the RGB% levels *will* be relative to the display's actual primary colors, if those have been measured. If they have not been measured, the RGB% are relative to the color standard that is in use (HDTV, NTSC, etc).
However, the end result is *exactly* the same. When you get to 100%, 100%, 100%, you have reached the target (i.e. "D65"), no matter what set of primaries were used to calculate the RGB percentages.
I realize the differences aren't huge, but if the purpose of the RGB percentages is to adjust the RGB gain and bias controls to calibrate the grayscale, it makes those RGB percentages less accurate to calculate them relative to an idealized set of RGB primaries (even if the errors are small).
The idealized primaries are only used if the "real" primaries are not known. It's a shortcut designed to get you into adjusting the gray scale immediately if you don't intend to (or can't) measure the primaries.
The only advantage for using some arbitrary set of primaries is that the user doesn't have to take time to let the measuring instrument "learn" the actual primaries. That's reasonable because the errors are usually pretty small...
Now, a completely different question is how the "Constrain Colors" option works.
The "Constrain colors to gamut" option in the Preferences creates an artificial "wall" at the edges of the color gamut (think WWF), and will not allow any colors to stray outside of this playground. If a color is measured and found to be outside of the gamut (theoretically impossible, but practically actually somewhat common), then that color is relocated to the closest point which is within the gamut (actually, on the very 'edge' of the gamut).
This feature was mainly for a customer in the Aerospace industry who wished to prioritize mathematical accuracy and purity over empirical data integrity.
Leaving this option OFF will make the software always show the raw readings that the hardware is providing, even if that means that a measurement plots outside of the gamut (and causes the RGB calculations, etc. to break down).
This option has *nothing* to do with calibrating gray scale, unless your gray scale is SO far off that it's outside of your RGB color gamut!
I hope that helps!