As i understand it, you are creating this 3 column display.
the left and right column are fixed and the maincontent div is variable.
If that is correct then why not simply use the max / min-width selectors in the maincontent div.
max-width:80% /*assuming left column is 20%*/
This should also work if you want to set the widths to either em, px etc.
see here for further explanation: http://reference.sitepoint.com/css/max-width