interesting-
I took a 1780 sq in CAP aerobatic model - which weighs 22 lbs , which calculates (according to the cubic ft calc) at 8.1.
You say that is good for a trainer .
That specific model actual practice-it is really an average aerobatic model, the wing loading being aprox., 28.5 oz/sq ft
To get into the "sport model " catagory (13--)as you noted, -it would have to weigh 37 lbs . I used the calc you furnished.
What did I do incorrectly?