title: Fixed position of button in the middle of fixed page
body: <p>my page has one big div with fixed width, like this:</p>
  
  <pre><code>#index_body{
    width: 1010px;
    background-image: url('images/main_bg_dark.png'); 
    margin: auto;
    overflow: hidden;
    min-height: 50px;
    padding-bottom: 10px;
    border-radius: 7px;
    -moz-box-shadow: 0 5px 15px #000000;
    -webkit-box-shadow: 0 5px 15px #000000;
    box-shadow: 0 5px 15px #000000;
  }
  </code></pre>
  
  <p>I want to add button (20x20px) on the right side of page (vertically in the middle) - still next to index_body.</p>
  
  <p>So the button has code, like this:</p>
  
  <pre><code>#butt {
    width:20px;
    height:20px;
    background: url('images/scrollUp.png');
    position:fixed;
    top:50%;
    left:WHAT SHOULD BE HERE??
  }
  </code></pre>
  
  <p>Because it depends on actual resolution. My index_body is always centered. if I change resolution my button is moved to the left-right...</p>