How does Agricola Solo version determine the food at the beginning of each game?
This is in the rules here

On page 9


  To play a Solo game, start with 0 Food. 


and later on the same page


  Because you have more permanent Occupations after each
  game, the goal score that you must reach goes up in each game:
  In the first game, your goal is 50 points, then 55, 59, 62, 64, 65,
  66 and 67 points. After the eighth game, the Solo game series is
  over. (You can of course play on with all permanent Occupation
  cards, in which case the goal score increases by 1 point per
  game.) At the start of each game in the series, you receive 1 food
  for every 2 points by which you exceeded the goal score for the
  previous game (rounded down).