Wander
The forward and aft CG limits will be CHOSEN by the designer while the aircraft is just a doodle. The role of the aircraft largely determines how big a range is necessary to make the thing a practical aircraft to operate (ie how it will tolerate in your case pax moving around, where the freight is etc. the military tend to want to carry stuff and then drop it and so on)
Having chosen the range you then need to design the longitudinal aerodynamics so that the thing will handle satisfactorily at both extremes. Don’t forget to mention to your stressman what download you will need at the tail for the forward CG case because this will likely be his worst shear case at the backend of the fuse when pulling to the aircraft g limit.
JF