I believe that the 'graphical' method to determine the weight that can be added, can only be used when the CoG position moves in a non-linear manner. So basically if you have a forward CoG problem with the echo, you need to use the graphical method. If you really become stuck, the beauty about the CASA exam is that you can check all 4 multiple choice answers to see which one is in ballance - and then you have your answer.
Cheers