According to Nathan's research, the FH steel used for the construction of Hood's belt was similar to that used for Nelson class.
Thorsten has provided us with a very nice primary document, which compares the protection offeered by "Nelson armor" (pre-1930s CA)versus "KGV armor" (post-1930 CA).Teh equivalent quality, depending on the thickness of teh material used, is between 85 - 95%, with the thicker plates being closer together in terms of comparative protection.
Thus, the 12" portion of Hood's belt was about 91% of the equivalent quality of KGV armor, which is very close to Krupp post-1930s CA, or 10,927"
I would be very interested to read more about these improvements in CA armour quality. Honestly spoken, I doubt that such a rapid improvement went on unnoticed by the british, who testimonied that rapid progress was made in their face hardened armour quality in the decades before 1910 but little progress appeared in the decade between 1910 and 1920, in which they were merely exploiting their findings.
I can guess about where it comes from. We have little cross country data aviable in this timeframe. The BADEN-trials (not those against the ship but those against some armour plates removed from the ship), evidence from Jutland, some russian AP and on british CA trials, the Ostfriesland trials with pre-1910 german KC against various U.S. Navy APC and the Indean Head trials of 1921 with US and british APC versus various US class A armour.
The HMS HOOD plates were ordered in 1916 and were not different to plates installed in the REVENGE class battleships, in RENOWN and REPULSE as well as in the QE´s and possibly in TIGER, too.
The Indean Head trials suggested that an 11 to 13% advantage exists in striking velocity for british plates over those typical for US manufacturers. Similarely, the BADEN trials show a very drastic inferiority of the KC plates (up to 30%) but this was not substantiated later by the real BADEN trials nor earlier by real penetrations through medium armour (5in to 9in british CA) at Jutland (or else the german L3.2 APC was much more effective in penetration than the Meppen trials or penetration graphs indicate: 0.5 cal at 500m/s and 30 deg obliquity, a possibility for which I don´t see any reason to accept).
Finally, the doc posted by Thoddy doesn´t only give relative figures for NELSON period CA and KGV period CA. It also gives average striking velocities at various conditions for KGV period armour and these need to be put in context with the ww2 trials on german improved KC/n.A. armour plates. It appears that the british trial plates comparatively tested against KC in 1946 were significantly better in resistence than average KGV plates guessing from the numbers of the original document comparing KGV and NELSON armour. It´s probably another case of selective comparison, which is my explenation for the BADEN tests, too. Anyway, german KC/n.A. was about as effective as the best quality 1946 british CA in thicknesses around 520 lbs in proper tests against 15in and slightly superior in resistence to average british CA used in KGV-class battleships. In tests conducted against 14in the british 480lbs trial plates seemed to be superior, but once more, these specially selected plates were also significantly superior to the figures cited for average british CA as of KGV quality.
The bottomline is I don´t think that HOOD´s CA is anything close to improved post 1930 KC/n.A. in terms of stopping power -it used an inferior steel quality, missed a proper temper and had a deeper chill than post 1930 british CA, causing larger scaling effects than ww2 period british CA. The latter was still using the same steel but tempered and had a much thinner chill, which greatly enhanced it´s stopping power against large projectiles. German KC/n.A. deep chill suffered from larger scaling effects but nevertheless was about as good as average british ww2 CA when attacked by major calibre APC owing to it´s superior steel quality and finer grain. Against ww1 period CA this should be quite significant because the temper advantage and the difference in chill depth beeing much less pronounced.
the U.S. Navy postww2 tested CA from Britain and KC/n.A. was found to be of about equal superior quality when attacked by large calibres (14in & 16in) compared to their own ww2 type class A (8.9 to 15% advantage) but only KC/n.A. was slightly better when attacked by 8in compared to US class A (3% advantage for KC as opposed to a roughly 10% disadvantage for CA over US class A).