I finally borrowed a better digital camera than mine and used
the zoom photo/Photoshop method of calculating frontal area:
Except the image I was actually working with was 1545x1127 pixels.
My results:
1.81 square meters / 19.45 square feet
This is pretty darn close to the Geo figures of
1.85 m^2 / 20 ft^2 listed here:
http://www.mayfco.com/geo.htm (which are uncredited, and tainted by the author's admission that many of the figures on his site are estimates).
Of course I only have one mirror, and much of the time the single mirror is folded flat, so I'll prep a few more variations of A for use in my calculations.