It is also worth noting that of that 900 million acres, only a portion of it goes towards food for humans or livestock. Most of the corn and soybeans grown are used industrially for ethanol, lubricants, acids, starches, and rubber. We also grow things like tobacco, medicines, and cotton, which may be needed, but are not food. A significant portion of corn is also traded as a currency “commodity crop” in a market I don’t understand at all.

If I recall correctly, roughly 40% of farmland is dedicated to food production for livestock, and 2% is dedicated to food for direct human consumption. So assuming you already have all necessities, and only need food without any change to an average american diet, we may need as little as 1.05 acres per person, following your math.