added option to switch off fast calc

rowers/templates/otwsetpower.html

@@ -35,6 +35,10 @@
 	We use FISA minimum boat weight and standard rigging for our calculations. 
 	</p>
 
+        <p>The Quick calculation option potentially speeds up the calculation,
+          at the cost of a slight reduction in accuracy. It is recommended
+          to keep this option selected.</p>
+
     <form enctype="multipart/form-data" action="{{ formloc }}" method="post">
     {% if form.errors %}
         <p style="color: red;">