My template is set out as follows;
The 'container' div (750 px wide).
1) inside this:
a) the 'main' div (500 px wide, float: right),
b) the 'sidebar' div (200 px wide, position: fixed, float: left).
2) inside the 'sidebar' are the 'title' div, the 'navigation' div, and 'description' div, the 'image' div, and the 'footer' div. The divs inside the sidebar do not have 'width' values.
I believe this CSS centres the page when applied to the containing 'container div': { width: 750px; margin:0 auto; }. If this is incorrect, I hope someone here will point me at the correct method.