Getting even light on the background

[Phtoo]

Hi when shooting against grey im having trouble getting the background/floor evenly lit. with white its easier because i can light it separately. but im not sure what to do when its grey.
if you look at something like this, http://payload.cargocollective.com/1/4/142428/2048609/HBALINE2_800.jpg the background is evenly lit. with a shadow on the floor.
Is it just lit with one source? im assuming my issue is with the distance of light to model , and model to background? any ideas, or online videos that might help me? is this inverse square law stuff??line videos that might help me? is this inverse square law stuff??

 

[WhiskeyTango]

It looks to me like there's probably a single light high camera right and a reflector camera left. I don't think there's any light directly on that background. Depending on how far the model is from the background, and how far the light is from the model, you might even be looking at a white background here...

You can make a white background go gray (or even black) by getting the lights much closer to the model than the background. If your lights are 2 feet in front of the model and the model is 6 feet in front of the background, that puts the lights 4 times further from the background than the model. That means 1/16th as much light hitting the background...