[MattKing]

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.

Eddy:

Correct me if I'm wrong, but I believe that the lighting setup you describe actually creates a 3:1 ratio.

In order to end up with a 2:1 ratio, one needs to have a fill light and a main light that, individually, provide the same amount of light.

That way, the shadowed side of the subject receives one unit of light (light from just the fill), whereas the other side receives two units of light (light from the fill, plus the same amount of light from the main).

I believe