The answer is 'too much'

Without being able to choose what cities map resources flow to, acheiving a level 5 city typically requires building the grain thingy wonder. The total required food should be 700 not 800. Plenty of cities I have built have been unable to breach the former value. It would result in more than 1 level 5 city, and perhaps growth should be slowed as that city reaches it's population limit, but it's better than being forced to build a wonder for it.

Additionally gentle rain should be avialable as a technology not a spell school, to illiminate the randomness of being hosed out of 20% more populous cities.