Calculation of the center of gravity (CG) of an aircraft model

Equations
C 
= (S(A+2B)) / (3(A+B)) 
MAC 
= A(2(AB)(0.5A+B) / (3(A+B))) 
d 
= (2Y(0.5A+B)) / (3(A+B)) 
CG 
= %MAC B.P.*(MAC) + C 


How to check CG
A correctly balancing rc airplane, sitting on your fingertips, will either be level or have the nose pointing slightly downwards. If the tail points downwards then the plane is tail heavy and you need to do something about that.
If the balance does need to be adjusted to get the correct Centre of Gravity, the first thing to do is try moving the battery pack further forward or backward inside the plane. By doing this you are adjusting the balance without adding extra 'dead' weight in the form of ballast.
Obviously you need to move the component in accordance with how the plane hangs: if it's tail heavy then move the component forward and vice versa.