Quote Originally Posted by markbarendt View Post
Actually, IMO there is absolutely no point in figuring or finding an average reading.

The lighting on the main, the most important side of the subject, needs to match the camera setting.

That camera setting places the important parts of the subject's "zones and tones" in all the intended places.

At that point your camera and main light settings, are "etched in stone".

The measurement and adjustment of lighting on the the other side of the subject is done "after-the-fact" to control the ratio.
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".