ORIGINAL: Lnewqban
Puff,
For a symmetrical quadrilateral, like your Straight Tapered Wing is, the span is irrelevant.
MAC = (12 + 6.75) / 2 = 9.375 = 9-3/8''
MAC LE location measured aft from wing's root LE = (12 - MAC) / 2 = 1.31 = 1-5/16''
0.25 MAC location measured aft from wing's root LE = (MAC / 4) + 1.31 = 3.65 = 3-21/32''
MAC is the chord of an imaginary rectangular wing that accomplishes the same aerodynamic function of your Straight Tapered Wing (equivalent area and lift).
0.25 MAC is considered the neutral point of the MAC (where the lift force is applied).
Those are your fix numbers.
The CG location is more flexible and depends on many other factors and preferences of pitch stability degree, as explained above.
Sorry, Lnewqban, you are wrong there.
What you have defined is NOT the MAC (= Mean Aerodynamic Chord)
You have calculated the Geometric Mean Chord. The MAC IS INDEED 9.625" as calculated by Puff right at the start, though as BMathews was right to point out, wings do not have a CG, that belongs to the whole aeroplane. The CG position will depend greatly on the stabiliser and fuselage, and any other bits attached thereto.
The MAC is defined in Aerodynamics as the integral of c^2.dy over integral c.dy but is easier to find by that graphical method shown