All good points, but I want to add a few.
1. A settlement's food storage should be determined by recent harvest.
2. And it should be influenced by the season when the enemies invade. For example, if an enemy army besieged one of your city in autumn and you cannot relieve it in time, then this city's autumn harvest should be lost, most grain will be reaped by enemies, that should hugely reduce the time the besieged city could last while prolong that of the besieging army. In fact, I remember reading quite a few of such cases in first chapters of Livy's History of Rome when Rome was still a small city/village, and waging minor wars with her neighbors.
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