According to the oz's your calculations would be correct, but I don't think there is 1 1/2 cups of jello in the bigger boxes, am going to have to go measure them, Know there isn't likely to be 3/4 cup in the smaller box either. If there is, I think my boxes have been falling short. That having been said, your Amish label should give you directions as to how much to use for a batch of jello. At least our local Amish or Mennonite store does on their labels, but can't tell you off the top of my head how much it is and don't think I have any in the cabinet at the present either.
My two cents for the day!