RealGoober> GA Adds only shield and trade, not food. You don't need an aqueduct to grow to size 12 when you have access to fresh water - i.e. lake or river.
At size 3, it produces 12 food and eats 6 food, so there is 6 excess food. I assume you have food storage to start with and it grows the next turn.
At size 4, it produces 15 food and eats 8 with excess of 7. So it will grow in 3 turn.
At size 5, it produces 18 food and eats 10 with excess of 8. It will again grow in 3 turn.
At size 6, it can only still produce 18 food due to only having 5 flood plain tiles and will eat 10 food. So, the excess is 6. You grow in 4 turns.
So, 1+3+3+4 = 11. You grow 4 pop in 12 turns.