I use this simple procedure to measure the surface area of a wing panel, its fairly accurate.

Root wing chord LE to TE = RC

Wing tip chord LE to TE = TC

Wing panel length = PL

Example :

RC = 20"

TC = 8"

PL = 60"

RC + TC divided by a factor of two = 14, for two dimensions. You can use as many as you want if you prefer to measure the wing in various places. This will provide the average chord length.

28 divided by 2 = 14

14 multiplied by the wing panel length = the average surface area of the panel, or 840 square inches...

Multiply by two for two wing panels, 1680 squire inches actual surface area, then allow for the width of the fuse at the wing root area or 60 x 2 + 8 = total wing length or 128 x 14 =1792 total surface area.