That's correct only if you don't have horizontal borders and padding on your sideboxes. If you do then the correct formula would be
column width = sidebox(width + left padding + right padding + left border width + right border width).
This is not actually what the default Zen Cart templates do. They do as you have. But if you look carefully at a default installation of the Classic template in Firefox you will see that the space to the left of the left column sideboxes is slightly bigger than the space to the right of the right column sideboxes. If you increase the borders on the boxes, the difference becomes even more obvious.
You won't see a difference in Internet Explorer, but there is one. In IE the column widths (which are actually table cells) expand. So the gaps remain constant, but the whole site width expands a little.
Even though the browsers do different things, neither is wrong. The error is giving them inconsistent information, i.e. a cell width and contents which are wider. FF places top priority on the cell width, IE on the width of the contents, then both do their best to display what has been given to them.