Why 9.2 inches is the cruiser sweet spot

What would be the ideal gun calibre for these cruisers?

Nine-point-two inches: it is the heaviest gun that still retains a markedly better rate of fire than the 10- to 12-inch class.

A 10-inch gun enters a slower firing-rate class comparable to 12-inch weapons, making it an awkward compromise. A 9.2-inch gun retains a rate more like the 7.5- and 8-inch class. It is the heaviest shell available before ammunition handling crosses into the slower category. Choosing the largest weapon within that firing-rate band gives a cruiser a more useful balance of shell weight, battery size, and volume of fire.