By default, root-level items are 124px wide. Multiply that by the total number of menu items to establish an overall root width. In this page's case, that would be:
4 * 124 = 496
Add a few pixels to accomodate borders and/or browser rounding errors. We wrap the menu in a container, ID="menubar" and give it a width of 500px, then use auto left and right margins to center it in the table cell. Here are the rules:
#menucell {
background-color: #ECE9D8;
border: 1px solid #333333;
}
#menubar {
width: 500px;
margin: 0 auto;
}