In your simple example, the wing chord would be 6", as that measurement is from the center of each radii.
However, consider the case of a cambered wing (sorry for the image from Wikipedia):
The wing chord is the straight-line distance from point 2 to point 7; you can see that this cannot be accurately measured with a caliper.
GA
On edit: this image is from this reference: http://en.wikipedia.org/wiki/NACA_airfoil
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