I am studying the Virtual Art Academy painting course, a good source of information and a community for serious artists. It is very interesting a deep a sort of main plot for future online campus like this one. Barry John Raybould the main master teacher made some good consideration after I posted a study made with Red Shift Engine. The study is fast but the Barry's notation are linked to the reality of drawing and are really inspiring.
Studying the core shadow in a light box
In this setup for learning how to shade geometric forms, there is no reflected light on the geometric objects. This makes it difficult to study the planes of the light and shade. You should arrange your setup so that there is some, but not too much, reflected light and a clear core shadow.
The core shadow line is a very important line to find when drawing or painting the portrait or figure because together with the outside contour, it helps define the forms. For this reason you should study it carefully, and learn how to define it subtly, without overemphasizing it. [comment by Barry]
Place the half tone with the light planes not the shadow planes.
Look closely at this half tone. It is to the right of the line that divides the light and the shade, showing that it is clearly belongs to the light planes, and not to the shadow planes.
Change of value away from the light source.
In this lighting example, the value reduces slightly in the light plane from the top to the bottom in the direction of the arrow. This is because the top is closer to the light source.
However note that the half tone at all times stays with the light planes, and never approaches any of the values in the shadow planes.