Quote Originally Posted by MattKing View Post
One point, for clarity.

If the light source illuminating the shadows on one side of the subject is in the nature of a diffused "fill" light, it will also be contributing illumination to the other, highlight side. Thus a change to that fill light will not only change the ratio, it will also necessitate a change to the camera exposure settings.

So it is important to differentiate between lighting setups that have two sources that illuminate separate sides, from lighting setups where one source is more of an all-over "fill".
Theoretically true, but the real effect I've seen in practice is limited.

I do normally take a final reading under the nose and that combined reading may bump up a 1/3 or 1/2 a stop. For most all my setups that bump can be ignored.

YMMV