I basically used the only available window that I have where I can make this kind of a setup. Direct light doesn't usually reach it so I had to work with soft shadows. Achieving somewhat of an adequate likeness was difficult as always, but I think it's kind of alright.
Ok so now for the thoughts:
I was actually surprised, because I didn't expect such a big area to be covered by the mass light. I had some misunderstandings about how some areas should be affected by the light like the area around the cheek bone for instance, but actually when I look at it by using the planes it's absolutely logical.
At some places it's extremely subtle and that's of course because of the diffused light source I have. I checked the values of the halftone and the reflected/bounce light and indeed they were very similar.
Basically the areas of the nose, the eye, hair and barely at the lips. Again normally when drawing a face I would imagine also a highlight at the cheek bone, but this study proved me wrong.
At first I thought that the area beneath the chin wouldn't be so dark, but think in planes again neither the reflected light from the wall nor the skin of the neck can direct any light to that area. Maybe if the chest area was exposed and directly under the light source then this would produce some bounce lights, but at current state no way.
So the reflected light as mentioned comes from a white wall near my face. It's really interesting that you can get this amount of reflected light with such a weak diffused light source.
Visible beneath the chin area, in the corner of the eyes and mouth and also slightly between the mass light area and the reflected light area. I find it a really important part as once you have it really helps with showing the forms better.
Visible underneath the nose of course and also beneath my weird hair. The hair shadow, again, is something I don't pay much attention to when drawing from imagination but it really helps to sell the idea that this is actually part of the head.