Other than by trial and error, is there a reliable way to calculate the maximum width of a background image for the main page? I know that the overall width is 770 pixels, and, from what I can tell, sideboxes are 150 pixels each. That should leave 470 pixels, before subtracting for borders and such. My background image is currently 450 pixels wide, but still seems to clip the left sideboxes when using Firefox or Opera.

Thanks,
Bob