4 fpt => 2 pop / 10 turns which is enough (actually too much).
Excellent rationale,
Erkon. Wrong, but excellent.
Here it is, folks, and listen carefully. ALL OF YOU, because this is also vital for WW.
HEAR ME? Straight out of
Obormot's mouth.
Poprushing 101
We do NOT need to grow our pop all the way up (to pop4, for example, if we have only 3 happies). No need although we can if we don't mind the angry face.
Note: This works when we poprush for 1 pop-point from almost pop3 and almost pop4. To poprush for 2 pop-points we either need to go all the way up to pop4, of course, or almost up to pop5, etc.
Example: To grow to pop4, technically we need 39

. With our granary, when we poprush down to pop3, the granary will keep half of 36

= 18

plus anything we have in excess of 18 up to 36

. So, to make the pop fall from almost pop4 (36/39

) down to pop3(36/36

) [ha ha], we need only 36

, not 39

(= pop4). Presto.
Erkon's Idea
We need 41

to bring our pop from pop2>pop4 and 41

for pop3>almost pop5 for poprushing purposes.
For two cities that's 82

.
4

surplus * 15 turns = 60

5

surplus * 15 turns = 75
6

surplus * 15 turns = 90

Bingo!
Each city needs 3

surplus, minimum. So with the banana farm and 2

in each city, technically we don't need a farm if we're poprushing units. The problem comes in when we need to grow the pop back up AND build a settler up to 61+ hammers for the poprush. For this, even with two farms and 1 unchpped forest still available, we can't poprush sooner than every 17 turns. Luckily , we only have a couple more urgent settlers to make.