alasdair

Yes, I just wanted to clarfiy that the MAC is not the same as the SMC or GMC that you used.
It makes little difference unless the wing is sharply tapered.

In Aerodynamics the MAC is the correct value of mean chord to use in calculations.

Just to enlarge on what you said, the MAC is the chord passing through the centroid of the wing shape, indeed the midpoint of the MAC (MAC/2) IS the centroid of a trapezoidal wing, or even a wing with 2 panels with different sweep and taper. The MAC does not even need to lie entirely on the surface.

A formula for the MAC of a swept tapered wing (or a panel) whose root chord is Cr and taper ratio is t (where t=Ct/Cr) is
MAC = 2Cr/3(1+t+t^2)/(1+t)

The two photos show the MAC of a two panel wing balanced on a steel ruler. Note that since it balances on the midpoint of the MAC, that must be the centroid of the shape.
Note also that the MAC extends slightly off the TE of the wing in this case.