title: Django decimal rounding
body: <p>I have this function and I want to allow only 2 decimal places for the value.</p>
  
  <p>This is not a model field but it's being calculated from them. What is the right way to do it?</p>
  
  <pre><code>@property
  def value(self):
      value = self.average_flow_rate * (self.d_n / 1000) / self.nu
      if 2320 &lt; value &lt; 4000:
          return 4000
      else:
          return value
  </code></pre>