I couldn't arrange the whole thing with one light source and a mirror next to a PC, so I did it in watercolours.
Took me a little more than 3 hours.
1. I noticed that mass light and shadow don't have to be equal in size, so to say - depending on light source, you can get lots of mass light or lots of shadow and very small areas of light. OK, maybe that was obvious.
2. Half tones in my case were much brighter than reflect light... I hope this is OK.
3. Highlights are not only the brightest part, but, on dark material (e.g. hair) are the places of the strongest contrast as well.
4. Reflect light gives you volume. Something I have heard lots of time, but really understood just now. With only light and shadow, the painting looks flat. With reflect light, it becomes three-dimensional.
5. Core shadow and mass shadow don't have to have distinct outlines - in my case, cast shadow from the nose blended with core shadow on the cheeck.