Theme width CSS

Hi. I posted this once and I didn’t get any replies so I had to figure it out myself.

I’ll change the theme width doing the following:
.fixed940 { max-width: 1200px; }
#core_columns_wrapper { max-width: 1200px; }
#core_left_column { max-width: 240px; }
#core_right_column { max-width: 240px; }

These being the following respectively:
Header width
Content area columns width
Left column width
Right column width

I’ve tested the above CSS and it works great, but I’m struggling to balance the middle column with those variables. If the total width is 1200px, the left and right sidebars are 240px each, then how would I code the reminder taking into account the left and right margins of the #core_middle_column? I tried #core_middle_column { max-width: 660px margin-left: 30px margin-right: 30px; } but it didn’t work.

And would these differ between the 2 and 3 column layout that I need to add a different CSS for each?

Any help would be appreciated.