agree 100% with JF but...
often the CG calculation, done by the weights engineer, will use fuselage stations or some other convenient geometric reference (since he may be having to consider the weight and position of things a long way from the wing, and doing calcs with "-150% mac" is a bit silly really.
So the weight and cg calc will consist of defining a whole bunch of masses at a whole bunch of reference positions (often aft of some notional reference, just in front of the nose, so that everything is the same sign). Then all the individual moments are calculated, and then summed, and then divided by the total mass to get a cg position, in the same reference.
THEN, to make it a useful number for pilots, aerodynamicists and other strange people, it'll be converted into a %mac (or something similar) so that it becomes a "sensible" number.