RE: Are My Calculations Right???
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.