Style Page Width Controller for Computer/iPad

I've been playing around with this and can not figure out how to make the following work...

I want at least 200px margins when on a "large screen", 940px minimum. When on an iPad, I don't want the margin on either side, just the content. Right now, when on an iPad, I have to scroll to the right to see the content (left margin shows, right does not).

Any help would be appreciated.