subject to white background question
Zlik is right, the exposure of the background will not change as you move the camera closer or fruther.
The background is a big light source. Where the fall off of light becomes important is with the amount of light coming from the background and hitting and wrapping around the edges of your subject. This can cause overexposure of the edges of the subject so that they blend into the background.
The further from the background the lower the level of light from the background striking the subject (inverse square law) and the less problem you will have with blowing out the edges of the subject.
I recommend you keep the background to no more than 1 stop of overexposure with the subject only 6' (2m) from the background. Personally I set mine to +0.7 ± 0.2 stops.
Theoretically, if you move the subject to 10' or 12' (3.5-4m) from the background you could get the same amount of light from the background wrapping around the edges of the subject with 3-4 stops of overexposure but then you start getting noticeable broad light source lens flare resulting in a loss of image contrast. I recommend you keep the background to no more than 1.5 stops of overexposure with a subject 12' from the background.
Living and loving it in Bangkok, Thailand. Canon 7D - See the gear list for the rest.