Just going by the EXIF names, I’d interpret those numbers as:
- Linearity Upper Margin: “from here on, signal is no longer proportional with light energy” (probably less signal than expected, and/or differences between the different colour channels)
- Normal White Level: “white surfaces (like paper) should be at or below this level”
- Specular White Level :“beyond this value you shouldn’t expect any detail” (i.e. the sensor is saturated). Somewhat higher values can still appear due to noise, and perhaps a small safety margin in the value.
That would also fit with the higher white points for the extreme ISO values, where you reach the 14 bit limit.
From this, and given the small difference shown between normal and specular white level, I’d use the specular white level as reported by the camera for the raw white point, assuming Canon (like other makers) know their own cameras… Assuming those values are fixed for each camera and ISO value, it’s not that hard to set up one or auto-applied presets for this.
And if you are doing extremely critical work, where linearity is of the utmost importance, you’d have to make sure all pixels are below the linearity limit (10 000).
All that of course once you have reason to think the defaults proposed by your program are wrong…