Using thick clear plastic, make a wedge about 6" long with a 20 degree angle. Mark a line down the centre so that you now have 10 degrees iether side of the line. Put your name on it so it doen't get nicked.
Makes it soooo easy to draw your ten degree fan lines on your chart - just place the centre line of the wedge on your plog line and mark the 10 degrees iether side.