A cautious estimate for Nagato's barbette width

What was the barbette width for the Nagato class's 16.1-inch guns?

An old note gives 24 feet excluding armor, but the missing citation makes that figure provisional rather than trustworthy.

An old spreadsheet entry repeatedly gives 24 feet excluding armor, but the source notes attached to other entries are missing here. The spoken reply also contains a conflicting 20-foot reference before returning to 24 feet. The figure is therefore an uncertain research lead and cannot settle the requested dimension. A forthcoming book of Japanese warship plans is identified as a way to check it.