Hi Stephen
My initial though is that you haven't allowed any space around the hugels for access. If you allowed say 4' alongside each
hugel (enough to run a smallish cart up between them to aid with a more effortless harvest), that would change your width to 12', so 300sqft, reducing your full acre output to 2,986,836.6 Cals. Not sure if this sounds more realistic...
Edit - it appears my thoughts have already been raised by others in a different time zone (pathetic excuse for not reading other comments first!)