In his book "Color & Light; A Guide For The Realist Painter", James Gurney explains how to simplify values. He shows an example of painted rocks, and explains how he simplifies their light/shadow into 4 shades - in reality there are countless shadows of varying degrees that would be too complicated to replicate, however his method helps to break down the process for the painter while still having enough variety to make it believable for the viewer.
When I took a printmaking class last semester, we practiced “aquatint” (a technique in printmaking that involves etching large areas of the plate instead of only lines) I was able to break up the values into 4 or 5 shades that I could work with. This made the piece much more manageable and interesting.
