For CSS changes, it’s a good idea to use a tool like firebug or Google Chrome’s built-in developer tools to locate them and then put the edits either in your child theme or in the “Custom CSS” option on your theme options page.
.double_sidebar #content { width:60.7%; }
.double_sidebar .fixed-sidebar { width:16.65%; }
1% + 16.65% + 1% + 1% + 60.7% + 1% + 1% + 16.65% + 1% = 100%
Also, here’s a helpful tutorial on how to use Firebug – http://vimeo.com/20917974. The only thing I’d suggest not doing is actually making edits to the theme’s CSS files, as it shows at the end of that video. However, the video serves as a good introduction to Firebug and how it can be used to find what CSS is effecting certain areas of your site without ever digging around in any files.