Thank you Hugo, I knew I had read it somewhere.
The size of the quad turret in width is the same width as the barbette (from photos and line drawings), which on SB original drawing is 81 pixels. Using 100" for the axes for the 15" this would give a width of 68 pixels for the triple 15". If anybody can give me a different figure I will happily use it.
Edit: This is the KGV with triple 15" using the above figures.
I am happy with the turrets, its the barbettes that I am not sure of.