In this shader, the lower scalar value is the median of the RGB components. So if the RGB values are RGB(0, 0.1, 0.5) the scalar value will be 0.1. This applies to white point, black point and gamma.
It's been about 16 years since I wrote this shader (can you believe it?!) but I think the reason it works this way is to allow you to shift the colours up or down without changing the spacing of the RGB values within the colour, and that can be helpful when dealing with negative black points. So let's take the example I gave earlier where the colour is RGB(0.0, 0.1, 0.5) and the median is 0.1. If you change the median to -0.4 all the values will be shifted by -0.5, so they become RGB(-0.5, -0.4, 0.0). This used to be a lot easier to see when the UI showed us all 3 RGB values (that changed before TG2 was released).
The number that's on the same line as the colour swatch works just like colours elsewhere in the program. It is the largest of the RGB components. But I wasn't satisfied with how that worked for black points.
Matt