Extend the centerline, aft from the trailing edge, a distance equal to the tip chord, to a point A. Extend the tip chord line forward from the leading edge, a distance equal to the root chord, to a point B. Draw a straight line between points A and B. Draw another straight line between the midpoint of the root chord and the midpoint of the tip chord. Where the two drawn lines cross is the centroid of the wing planform. Draw a chord through the centroid. That chord is the Mean Aerodynamic Chord (MAC). Project the MAC to the center line. The CG will lie somewhere between about 15% of the MAC from its leading edge to 25% of the MAC from its leading edge, depending on how much static stability is desired, and how much the CG shifts from a full tank to an empty tank. The "dirtier" the aircraft, the more forward the CG needs to be. Props, uncowled engines, landing gear, exposed control linkages, etc. require a more forward CG within the above mentioned range.