title: How calculate centered div's left side space
body: <p>I have centered div for my side. Now I want to calculate left side blank space. How to do it by jquery.</p>
  
  <h2>My Markup</h2>
  
  <pre><code>&lt;main id="page_main" role="main"&gt;
      &lt;div class="content"&gt;
      &lt;/div&gt;
  &lt;/main&gt;
  </code></pre>
  
  <h2>My Css</h2>
  
  <pre><code>#page_main
  {
      width:100%;
  }
  .content
  {
      width:960px;
      margin:0px auto;
  }
  </code></pre>
  
  <p>Now I want to calculate the screen's left side blank space. Can you help me how to do it?</p>