Let's say that in order to move back the Center of Gravity by 1 inch you need to add 4Oz at the butt, you could also add 8 Oz 1/2 way in the stock. Because the moment of inertia is proportional to Mass x distance squared the second solution has 1/2 the additional inertia. The penalty is that it also has more mass.
e.g. there is no free lunch.
Balancing should be static and dynamic and it is quite an art (that I know nothing about) on complex objects such as shoulder guns.
Attempting to measure things is a necessary first step in learning about this subject matter.
Best regards,
WC-