Ok but did changing the css accordingly give you the result you were looking for on the product details page? I know it messed up the other pages.
I am not the best with "if statements" but I can give advice on the psuedo code for it.
{if $main eq "product"}
Change style for margin on the center-main to 0 0 0 0, this is where you may want to hard code style, inline with the <div>
{/if}
You would put this code around the creation of the center-main div.
Mind you I have not done something like this, it would be best for someone else to help out figure out the rest of the code.