Not magic, it is only a tool, but a good one to use to judge, and compare every paint mix we make, to see where it fits in the gray range.
Let's start by making one.
The first time, I mixed, what I perceived to be a "centrist" or middle gray for the #4 box and mixed values up and down.
The second time, I started with white and added a little black to the mix as I worked up the scale to all black.
This is something that you get better at the more that you do it; so don't stop at just one.
Do several, set them aside and judge them later as to which seem to get the best gradation of even values up the scale.
Next we will look at just WHERE our colors fit in this scale.
A hah! More fun to come.