Looking at my palette you will see two dividing black lines and although they are reinforced post production digitally they were physically drawn in on my glass palette with a sharpie marker. This is actually the firt time I did this and I am please with the results. The point of these lines are to separate my tone into the corresponding groups as i mix. This helps the painting process because I can clearly see whether my mixtures belong to the light or shadow and then judge all new mixtures according to what I have already. This tight organization with translate into your painting. Using a 9 value scale I have limited myself on what values can be used in each group (fig. 1.) the tones used in each group can vary depending on what is in your painting but the driagram below illustrates a general plan for subject matter that has a full range of values in it. By full range of value I mean it contains something that is near black eg. a shadow on a dark object, some source of light eg a secondary light source or spectacular highlights and everything in between.
Notice how few mixes are needed to make a painting
Using this scale is easy, the lightest tones in the groups represent "white" and they gradient towards "black" at the other end of each group. so the first value in group 1 is a spectacular high light on a white object, the first value in group 2 is a area in the light on a white object, and the first value in group 3 is the area in the shadow of a white object. Similarly the second value in group one, the fourth value in group 2 and the third value in group 3 might represent a object who´s local value is a mid-tone.