I'm not a good at calculating ahead with settler factories ...
Ok, let me explain it. You want a settler factory to be in a sustainable cycle. That means roughly that you want to compensate the loss of 2 population (by creating the settler) by making your city grow 2 citizens in the time it takes to create the settler. The standard example is a town with a granary that can do 5f6s (= 5 surplus food and 6 shields) at size 4, and 5f7s at size 5, and has one extra tile that has 2 shields that is selected by the governor on the growth turn. In that case the cycle starts at size 4, the settler will be produced on growth to 6, minus 2 for the settler, and we're back to size 4, the start of the cycle. The cycle then looks like this:
Start of turn 1: size 4, 0 food, 0 shields.
Start of turn 2: size 4, 5 food, 6 shields.
Start of turn 3: size 5, 0 food, 14 shields.
Start of turn 4: size 5, 5 food, 21 shields.
Start of turn 5: size 4, 0 food, 0 shields ( = 30 shields, so settler produced).
The extra 2 shields in turns 3 and 5 come from the high shield tile that is selected by the governor on growth.
In many cases you will need to MM the settler factory on turn 3 and 5 and change the high shield tile for a 2 food tile. Also note that the city advisor does not account for the extra shields on growth, so it will tell you "Ready in 5", while in fact it will be ready in 4 turns. For me this last point was very confusing when I first tried to set up a settler factory, breaking my mind on the question what I was doing wrong, while in fact the advsior was doing it wrong.
I hope this all doesn't sound too pedantic. For our current game, this is Delhi:
Four a 4 turn settler factory in Delhi, we have at least 3 options.
We are at size 3 and are doing 4f7s, which is very good, but we miss 1 food for a 4 turn settler factory. If we exchange the forest-deer (2f2s) with an irrigated floodplain (3f0s) which will be ready next turn, we are doing 5f5s at size 3, and 5f7s at size 4, and then we do not have 30 shields in 4 turns (but 28).
Option 1. A solution could be to use a 3rd irrigated cow that is now used by Bombay for 1 turn at size 3.
Start of turn 1: size 3, 0 food, 0 shields. (Use 3 irigated cows: 5f7s)
Start of turn 2: size 3, 5 food, 7 shields. (MM: Use 2 irrigated cows, 1 irrigated FP: 5f5s)
Start of turn 3: size 4, 0 food, 12 shields. (Use 2 irrigated cows, 1 irrigated FP, forest deer: 5f7s)
Start of turn 4: size 4, 5 food, 21 shields. (Use 2 irrigated cows, 1 irrigated FP, forest deer: 5f7s)
Start of turn 5: size 3, 0 food, 0 shields settler produced). (MM to use 3 irigated cows: 5f7s).
Pros and Cons:
MM complex and involves both Delhi and Bombay.
We have no waste of shields
ETA of our first series of settlers: turn 3,7,11,15,...
Option 2. Use the 3 irrigated cows in the whole cycle. This also gives a cycle at size 3 and 4.
Start of turn 1: size 3, 0 food, 0 shields. (Use 3 irigated cows: 5f7s)
Start of turn 2: size 3, 5 food, 7 shields. (Same)
Start of turn 3: size 4, 0 food, 18 shields. (Use 3 irrigated cows, forest deer: 5f7s)
Start of turn 4: size 4, 5 food, 25 shields. (Same)
Start of turn 5: size 3, 0 food, 0 shields (34 shields = settler produced). (MM to use 3 irigated cows: 5f7s).
Pros and Cons:
MM is much less, since likely the deer will be chosen on growth to 4, so we only need to MM on turn 5.
We have 4 shields waste.
Bombay looses a good tile.
ETA of our first series of settlers: turn 3,7,11,15,...
Option 3. Use a cycle at size 4-5, with 2 irrigated cows, an irrigated floodplain, and the forest deer.
Start of turn 1: size 4, 0 food, 0 shields. (Use 2 irigated cows, deer, irrigated FP: 5f7s)
Start of turn 2: size 4, 5 food, 7 shields. (Same)
Start of turn 3: size 5, 0 food, 18 shields. (MM: Use 2 irrigated cows, deer, irrigated FP, some other 2 food tile: 5f7s)
Start of turn 4: size 5, 5 food, 25 shields. (Same)
Start of turn 5: size 4, 0 food, 0 shields (34 shields = settler produced). (MM to use 2 irrigated cows, deer, irrigated FP: 5f7s).
Pros and Cons:
MM at turn 3 and 5.
Need an extra MP (but if we choose this option, we can dump the shileds we currently have in a warrior).
We have 4 shields waste.
Bombay does not loose a good tile.
We are larger so make more commerce once all tiles are roaded.
ETA of our first series of settlers: turn 5, 9, 13, 17, ...
I'd like to have your opinions on this. Option 1 is theoretically the best (no waste) but I've seen the MM bit of a settler factory go wrong in so many SGs that I actually vote for option 2, since if I'm correct it will only require MM on turns that the settler is produced. Option 3 is fine too but it delays our settler series by 2 turns.