Ok, let's do it by hand, and then check the calculator.
So, in cubic inches the mold is 81. To get the amount of oils for that volume, that's multiplied by .4. So that gives us 32.4 ounces of oils.
That's exactly what the calculator gave me.
Are you reducing the amount of liquid? What is the ratio of water to lye you're using? I often use 2.5:1, which would mean adding 14 ounces of water. So I"d have a total, roughly, of 46 ounces of batter. That doesn't include the lye and/or fragrance, of course. So maybe I'd go with 2:1 water:lye in this case.
If you're short, maybe it's a problem with some of the measurements. I weigh my ingredients by ounces first, but then double check with grams as well to make sure my measurements are correct.