/**
  * Calculates the appropriate y position for the bottom of the columns on this page.
  *
  * @return the y position of the bottom of the columns
  */
 private float getColumnBottom() {
   if (desiredHeight == AUTOMATIC) {
     return document.bottom();
   } else {
     return Math.max(top - (desiredHeight - totalHeight), document.bottom());
   }
 }