title: Calculating height of item relative to location
body: <p>I have an item which needs to be of height H1 at point P1 and H2 and point P2.<br>
  At any point between P1 and P2 the height needs to be somewhere between H1 and H2 relative to its distance from P1 and P2 (on straight line between them).</p>
  
  <p>E.g.:
  If H1 = 100 at P1 and H2 = 150 at P2, then on the mid-point between P1 and P2, the H = 125.</p>
  
  <p>How do I calculate this?</p>