At those speeds I wouldn't go any lower than 85-90 sq in wing. Mine has 106 because it's an old Q-Rat (and it flys great). If you don't have the lift it'll fly nose up and just plow through the air. I learned that making 22" span slow rat wings. More span gives you more lift too.
Anybody want to write a speed/weight/area/span/airfoil thickness app?
MM