I don't know, I'm running 9.9" on my L1 and as long as I manage to slip through max
range of those 45" guns I seem to do fine. I can surely last a lot more than without
armor and 3 turrets. Sure, having armor doesn't mean you're invincible but it's
sometimes laughable seeing a couple bb3s shoot at you at their mid angles and getting
each shot bounced.

It is not really how much deck you can mount, its how you play an AW ship. Anything 7" is
usable, 9"+ is preferable on higher tier BBs.

The key point of armor is, do not let yourself be hit at max angle by a IJN BB3+ US BB5+
or L2, played well, you can get away with bouncing all of those ships.

It always depend against what you are fighting. 9inches on my L1 gave me excellent
protection against bb3 and under, and KM bb4-bb5. Iowas were sometimes bounced. Against
the other BB4-BB5 the armor was useless. In fact, it was worst than useless cause it
slowed me down.

If you go AW pick your battles unless you are sailing with more than 11 inches of deck.

4.7 inches will do you no good!

Right. Not all UK battleships can effectively armor up. Lion, PoW, and Nelson are best
at it.

So it is pointless for my Dunk on which i can equip like 4.8 inches of deck?

Over 11'' deck.
Else,
It's pointless when a nagato, yammy,super yamato,L2,Monty laugh at your useless
armour if fire at max range.