In my simplified tonal value scale I mix the grey values and place them from top to bottom. Black is the starting point and I mix in more and more white. Now I compare which of the mixed colours correspond to the respective tonal value. The trick is that when I pinch my eyes together, everything blurs to one tonal value. (Note: I do not use black to darken the colors)
Detail: The Reader
Detail: Pastel picture
The whole principle also works with pastel, without having to use black for darkening. That depends on the manufacturer. Very dark colours are offered by Sennlier, Unison and Terry Ludwig, for example.