title: Sass calculate percent minus px
body: <p>I want to be able to do the following:</p>
  
  <pre><code>height: 25% - 5px;
  </code></pre>
  
  <p>Obviously when I do that I get the error: </p>
  
  <pre><code>Incompatible units: 'px' and '%'.
  </code></pre>