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It isn't just a case of just adding whatever is necessary to get 255, 255, 255

You're really looking for an affine transformation (i.e., solve for A and B in Ax+B), such that the selected pixel ends up grey, and so that you don't screw up the brightness/contrast of the image. Picasa has an "eyedropper" tool built in that does that. A harder problem is "automatic color balance", where you want to do this without an eyedropper tool (and, possibly without any reference white point anywhere in the image). Picasa's "auto color" or "I'm feeling lucky" sometimes works for me, but I'm a big fan of having a reference white point that I keep in my camera bag.