According to Daniel Glazman, the W3C CSS Working Group has finally accepted a proposal which will mean web developers will be able to specify the width of an element in a percentage plus/minus a a set number of pixels.
"So if everything goes right, we should have in the near future a mean to say "the width of this element is 50% of the parent box's element minus 12px" !!!! This is BIG, this is poweful, this is cool. Web designers, please stay tuned, the CSS WG hears you."
Have wanted to use something like this quite a few times; sounds great!