title: Responsive section of fixed element
body: <p>Total page width is 1200px. I have an element which has a width of 70% which would be 840px when max width is reached. I then have a left and right section in it. I want the left to stay at 40px and the remaining 800px to be responsive. </p>
  
  <p>Do I make the remaining 800px a percentage of the 1200px or the 840px?</p>