Bread, wood, stone and iron sit at 20 : 20 : 4 : 1. For every unit of iron you want twenty of bread, twenty of wood and four of stone. The game states the same ratio as a par value of 50 : 50 : 10 : 2.

Multiply your iron by twenty for bread, twenty for wood and four for stone. At 1.2M iron that is 24M bread, 24M wood and 4.8M stone.

Iron is the scarce resource: it is what you run out of first, so it is the number worth building the other three around rather than the one you discover is short after gathering everything else.

Your own stock, redistributed into 20 : 20 : 4 : 1. The four targets add up to exactly what you already hold, so one resource reading as over is what makes another read as short.