Tsingtao 10 BC
So let's look at Tsingtao.
Every citizen is producing two food and we have a surplus of two food each turn. In food production, all the tiles are equal. Only four of them are producing any shields; the tile SW of the city has no sheilds.
So if that citizen became a specialist, the city would be at zero growth but would still make a settler in 5 turns. In theory, that is.
Here that citizen has become a clown. The city is indeed at zero growth and the settler will still be completed in five turns.
Notice that our commerce has dropped from 14 to 12. Commerce is in the middle of the city screen, under Production and Food. Before we stopped our city growth, we had 14 commerce per turn; afterwards only 12.
Before, that 14 commerce was broken down into 6 gold, 3 beakers and 1 smiley, with 4 commerce lost to corruption. Afterwards, we had 5 gold, 2 beakers and 2 smileys (1 more due to the clown) and 4 commerce lost to corruption.
Here the clown has become a geek; we get a lot more science and the city stays productive and happy.
Canton 10 BC
Canton is not so easy.
Every citizen is making a sheild, so hiring a specailist will also delay the settler. The city also has a surplus of 4 food per turn; quite a lot.
If we hire the citizen working the hill, Canton grows in 4 and the settler completes in 7; 3 surplus food. If we hire the citizen on the unroaded plain, Canton grows in 6, but the settler still completes in 7; 2 surplus food. If we hire the oasis, Canton grows in 12 and the settler completes in 7; 1 surplus food; good. If we hire the wheat, Canton has no growth and the settler completes in 7; also good.
Slow growth is better than no growth, so lets make a specialist of the citizen working the oasis.
That is what Canton looks like after we make the oasis worker into a clown.
That is what Canton looks like after the clown becomes a geek.
That is what Canton looks like after 4 turns. It needs 8 food to grow (1 surplus food; 8 turns) and 9 shields to complete the settler (4 shields per turn; 3 turns). The Blue lines indicate where our food and shields started at four turns ago.
But, if we fire the geek and put him back on the oasis we are back to the initial situation, only on our terms. Now we have 4 surplus food and grow in 2 turns. We also have 5 sheilds per turn and the settler completes in 2 turns, with only one shield being wasted as an overrun. Growth and production occur at the same time, so no riots.
The real tricky part is remembering to make the geek into a working citizen in four turns.