I've got 10 lbs of FHM in a new 1/2 acre pond, accompanied by 24 adult RES, right now. They are producing an amazing amount of babies. But I think maybe I'd go 20 pounds for 1.5 acres.
BTW, I forgot to repeat this advice earlier - anytime stocking costs seem high, just compare them to the cost of building the pond. And DLYWKHMTPC. (Mine does)