function calculatePinchZoom(startDistance, endDistance) {
      var percent = (endDistance / startDistance) * 1;
      return percent.toFixed(2);
    }
Calculate the zoom factor between the start and end distances
@param {int} startDistance Distance (between 2 fingers) the user started pinching at
@param {int} endDistance Distance (between 2 fingers) the user ended pinching at
@return float The zoom value from 0 to 1.
@inner