The infants stuff is more concentrated than the childrens stuff. My DS is 13 months, 24 lbs and gets 3/4 tsp, so 1/2 tsp at 20 lbs sounds about right to me.
ETA: Here is the Dr. Sears chart. According to it, you could go to 3/4 tsp and my DS could actually get 1 tsp.
ETA: Here is the Dr. Sears chart. According to it, you could go to 3/4 tsp and my DS could actually get 1 tsp.