Hi Wargames.
Sorry for the delay in responding. This is not a straight forward question to answer. In my opinion, shipbuilding for most nations during 1920’s and 30’s was done under the umbrella of the Washington treaty which limited sizes. Thus it was a juggling act between various components of warship in order to get the best vessel within the limits of the treaty even though a lot of the vessels actually exceeded the limitations!
I attach a some pages from Bagnasco and de Toro’s excellent reference book ‘The Littorio Class.’ These pages deal with armour of the citadel. You may see that there almost a citadel within the citadel. The book details all the protection of the class including the turrets. There is far too much to reproduce here but I do recommend the book.
Having said all the above, I am not sure if I have answered your question as I cannot offer an opinion of other countries battleships.
Regards,
Bob
PS Gaving problems attaching photos. Will try after posting this reply!