Don't think of the background light as just a background light. Play with it.
If you double the distance you will diminish the fallout.
Your settings http://otake.com.mx/Foros/BackgroundIlumination-1.png
You will need to double the output, and probably put a card so you dont spill your subject.
Double the distance http://otake.com.mx/Foros/BackgroundIlumination-2.png
But play with the light, cut a cardboard in diferent shapes.
Play http://otake.com.mx/Foros/BackgroundIlumination-3.png
From diferent angles
Play http://otake.com.mx/Foros/BackgroundIlumination-4.png
Put a difuse before the cardboard, remove the difuser, move the distance to the screen, to the cardboard, the angle.
Take a time to shoot not your portrait, but to shoot just backgrounds.