My number should not have been that far off of yours. I found three things that account for that.I hadn't considered that "Non-married Family Households" might have two adults. My number is also low in that it maxs out at 7 people per household. You could use my estimate as a "minimum." Your's is probably more accurate (and it doesn't even include fraud!).
I think the $480 Billion would stand up in a court of law. Trying to make it anymore accurate is pointless, as errors in the census data in undercounting people and other types of error, puts any further adjustments in the noise.