title: Colour scale based on how far value is from other cell value
body: <p>I have a column of targets (<code>C2:C30</code>) and a column of projections (<code>D2:D30</code>). </p>
  
  <p>I've been able to conditionally format the projections to highlight values either above or below target, but I want to expand this into a colour scale to be able to identify more quickly how <em>far</em> from the target my projection is.</p>
  
  <p>For example, if <code>D:D</code> is above 90% of <code>C:C</code>, a green colouring would suffice. 80-90% would be amber, below 80% would be red. In an ideal world, a fully fledged colour scale would be great but at this moment in time I'll be happy with anything more sophisticated than my current above/below setup. </p>
  
  <p>I believe I could use a series of stacked conditional formats but thought there might be a more efficient way of doing this?</p>