Hi Merv,
it is all to do with position and f. numbers.
As an approx. guide the white BG should be lit approx 2 stops faster than the subject. The lighting should be arranged so there is no spill of light to fall on your subject from the BG light.
eg: If your main subject (model) is to be lit at f.5.6 then the BG should be lit at around f.11 This means that the BG light is 2 stops brighter than the subject. Once you have established the correct light falling on the BG then forget it and set your camera and main lighting for the main subject, eg. f.5.6 (This may sound confusing, but it is not really) LOL
attached is a very rough guide drawing, I hope it helps

Attached Images

lighting.JPG (148.2 KB, 19 views)

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Regards....Phil

If it moves shoot it. If it doesn't move, still shoot it.