I know some people want a wider template than 960 so I made an excel spread sheet that should give you the correct variables for your CSS files. Just plug in the width in the cell C1 and it should give your the numbers for your css file.
So you can resize to:
1020px
1200px
etc
Hope this helps. I believe everything is right if not let me know
In the member submissions you find a gantry-feature that does the calculations automatically and adds the css (based on the width of a single column-width wich is 60 by default).
Should be also noted that you should never change something in com_gantry. Add your own css files via the gantry-method ... like $gantry->addStyles(array('template.css','joomla.css','your-file')); for example ...