Your grain absorption rate is your water in less your water out divided by the amount of grain.
So in the case you mentioned:
Water in = 20.75 + 12.90 = 33.65 liters
Water out = 24 liters
net loss is 33.65 - 24 = 9.65 liters
Water absorption by grain = 9.65 / 6.4 = 1.507 liters/kg. Converted, this is about 1.44 oz of water per oz of grain (default for BeerSmith is 0.96).
Now, how confident are you in your water measurements? Are there any other losses that need to be taken into consideration? Your loss is about 1.5x what I would expect from a standard mash tun set up and there would be a lot of free water (wort) coming out when you remove the grain canister from the Grainfather. Rice hulls will not absorb much water and a finer crush _could_ be a small factor, but it would not account for such a large amount of retained water.