Try not to complicate the thing too much, you want the answer in ounces per square foot. So calculate the weight of the model in ounces, use an imperial scale and weigh the thing, and convert your answer to ounces if you need to. If you calculate the wing area in inches, then you will need to divide by 144 to find the square foot number, easier to measure the wing in feet, and fractions of a foot. Then divide the weight (ounces) by the area (square feet). The math is essentially simple.
Evan, WB #12.