Yeah, 1 kg/day in modern times to maintain weight and full fitness sounds right to me. In prehistoric times, the average human was significantly shorter (so lower BMI) and the body will down regulate calorie burn as it enters starvation, so I guessed the minimum requirement during a prehistoric famine was about half, i e. 0.5 kg.
Discussion
IIRC humans weren't really much shorter before they started eating grains. Even with down regulation, I doubt they could go under 0.7.
Anyway, that supports your point - it's probably higher.