That works pretty well if your wings are rectangular (which most wings are.
)
I never meant to imply that this was
the way to compute surface area; it's just a trick I thought of that I sometimes find convenient and wanted to share. A lot of people have an old version of Photoshop sitting around.
For me, it's a way to take something like this:
and quickly see that, say, with a 24" wing span, that plane would have about 147 square inches of wing surface. With a 48" wing span, the wings will weigh just shy of 4 ounces if I make them out of Dollar Tree foam with the paper on, etc. etc.