The idea to give core cities 24 food in total, and then max the number of shields with mines is a good one.
The only exceptions: Overly corrupt cities should get maxed food, (irigate everything) to allow them to use specialists.
If you want a city to grow fast, give them more irrigations to speed up this process, change back to mines later.
In the late game, if you plan on a space ship win, you can speed up research at the cost of production by irrigating everything in your core an letting cities grow past size 20, as big as food allows. (though this is rarely used nowadays because of our understanding of specialist farms, and hyper aggressive conquest, in the past, when people still build OCP, even the pro's who played deity, this was more often used.)
I've changed some of my core cities to 1 turn worker factories when I needed more workers in the late game, or to speed up the growth of other cities to 1 pop per turn. So I build a granary and then irrigated to get exactly 20 food per turn surplus.
The trick is to grow as quickly as possible.
This is also complicated slightly by the despotism 'penalty'.
While you are in despotism Irrigating grassland does no good.
So the old rule is "Mine green and Irrigate brown".
This only applies before you change government and it DOES NOT apply to food bonuses.
So water the cows and wheat no matter what tile they land on.
I know what you are trying to say, but to be more clear: The despotism penalty does apply to food bonuses. Its just that when the amount of food becomes more than 3, irrigation will become useful.
Sugar or wine on plains also isn't helped by irrigation on despotism, and floodplain without any food bonus can break through the barrier with irrigation just like a wine on grass can.
