Ellis Vener , Apr 03, 2011; 10:56 p.m.
The secret is lighting in layers: separate lightign o nthe background from your the main light on your subject. With flash i nan otherwise normally lit room you don't have to worry about shutter speed. Ignore the ambient light reading unless that is your key.
Ideally you use a pure white background -- Savage Super White paper is the standard. You light it from both sides or if you have a boom and sufficient ceiling height, one light from above can work. If you are using an incident type meter tha treads the light falling on it, generaly you don't need the background be to lit no brighter than about a half to a full stop brighter than your key subject light -- a 2:1 ratio . Given that your background is around f18 and the lightr eading on your subject is f/2.8 your ratio is more like 30;1 and the background is effectively bright enough for your camera to see it as a light source. That is a recipe for detail destroying flare, hence the milky rendering of the subject. If I were were you, I'd get two AB400s and large softboxes, some large (48 x 98 inch) fomecore flats (white or black) and a good lightmeter tha treads flash.
I'd also get a copy of "Light: Science and Magic", third edition, and postpone your date for a week or two while doing some serious studying and experimenting. That will do a lot to allay your panic.