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".
True, and since most portraits fall in that category, it's a very simple process:
Turn on main light, point the dome at it and take a reading. Turn off main light.
Turn on fill light, point the dome at it and take a reading.
For a 2:1 ratio, fill should meter one stop less light than main.
Turn main light back on. point the dome at the camera, and take a reading.
Set the camera and take a picture.

More complex lighting setups will call for more complex metering procedures, hence the need for the flat panel.