0-Impacts of shells.
There are 2 types (main types) of shell impacts:
Deck-shots hit you on the deck of the ship. You can recognize them when white numbers pop-up when shells hit.
Belt-shots hit you on the belt of the ship. You can recognize them when green numbers pop-up when shells hit.
What determinates if a shell is going to hit in deck or belt? Well, there is certain randomness but it's mainly determined by the angle the shell was shotted. If it was shoted at high angle there will be more probability of hitting in deck and it it was shotted with a low angle there will be more probability of hitting you in belt.
What angle can be taken as a reference? Well, let's say around 20 degrees. But sometimes you can shoot at 30 degrees 9 shells and got 7 belt impacts, other times you can shoot at 15 degrees 9 shells and get 7 deck impacts. There is no a "hard" frontier, its random. I even have seen sometimes belt impacts shotting at 45 degrees, but its quite rare.
The damage of the shell also depends on angle. It works like the damage of a shell has a "kinetic" component and an "explosive" component. Belt-impacts will make more damage as lower the angle (impacts go more perpendicular to the belt of ship, kinetic damage is higher) and Deck impacts will make more damage as higher the angle (they land more perpendicular to the deck of the ship, kinetic damage will be higher).
As its name indicates it works against deck shots.
Obviously most of the hits a BB receive are on deck, it's a rare battle situation being fighting under 20 degrees.
Deck armor works the way or you bounce a shell (0 or very low damage) or you eat the full damage completly.
So putting small amounts of deck is completly uneffective, there are urban legends around putting a small amount reduces crew deaths. Everyone can believe what they want.
So, what is a "decent amount"?. Some references, they are just references:
With 5.0 on deck you can bounce CA (non-PCA) shells at max range, i think with this value you can bounce in deck any FF/DD/CL/CA ship. But well, that's not quite useful, isn't it?
With 8.0 you can bounce most of PBB/BB123 at max. It's just an aproximation, maybe there can expeptions like NorCar (45 degree guns) or Nagato (43 degree guns).
About BB4 the "magical" value was 9.1 UK deck armor. With that you can bounce every BB4 shells at max range except Yammy and Stalingrad.
About BB56 with around 11.0 deck a QV can carry you can bounce KM BB56, MN BB56, UK BB6 and RM BB56.
So, with those references it's up to you to see if you want to put deck armor or not, but i think is fair to say in GB is a bit stupid to put less than 8.0.
As it name indicates it works against belt shots.
The effect of belt against shells is completly useless, so putting belt for that is a nosense. Very few ships can hold large amounts of belt. The only ships where this is good i found where Prinz Eugen and Omaha on blitzs.
The most important effect of the belt in game is that putting 0.2 on belt halves the torp damage.
Over that amount there is no significative reduction.
So, put 0.2 of belt in all your ships, its not a recommendation, it's a must.
When your DP bar is under certain frontier value you loose overheat capability.
That value is determinated with the "defense value".
Putting Bulk Head increases that value, allowing your life bar go lower without loosing overheat capability.
Unfortunately this armor weight is too high and the effect on that is too low. Both things. So it's completly uneffective.
Don't put Bulk Head armor on your ship.
Bulge works against torps.
This is the only type of armor that is "spent" when you receive impacts.
IJN bulge was the most effective if i remember well.
Configuration of the 99% of the ships.
-Put all your sailors, guns, engine, FCS, HHs, shells and everything on your ship.
-Go to armor panel.
-Put 0.2 belt.
-Put the rest on bulge until you loose a knot, then go back, recover that knot and press ok. If you are a CV maybe you would want to sacrifize some knots in exchange for more bulge.
Configuration for deck-armored ships.
There are people that puts 0.2 belt and the rest of displacement on deck and there are people who put a random amount of bulge, like 50 or 100 points and the rest on deck.
Apart from 2-gun L1, 2-gun L2 and QV there are no much more ships where putting deck armor is effective.
3-HE and AP use.
By default, against an unarmored target you make more damage with HE ammo, so using AP on unarmored ships is stupid. So, mixing AP/HE in the same salvo is completly uneffective and it's amazing how there are still people doing that.
So for using AP you have be aware of:
-If the target is a 2-gun L1, 2-gun L2 or QV by default that means is an armored target unless it goes too fast.
-If you have a BB456 there are only (mainly) those 3 ships to have in mind, so it's no so hard to remember them.
-You have to be aware of the angle and the shells you are shotting. They will certainly not bounce yammy,stalin,L2,Monty,P24,Nebby shells at max range, but if they are inside your range some degrees you have to use AP because they will bounce them. For the rest of the BB456s (if i didn't miss anyone) you can put directly AP.
-And well, there can be multitude of battle situations. For example some people armor L2s with 3 guns (like, idk, 7.0 on deck or so). If this ship is very inside your range (angle 25-30) he will bounce your HE shells, no matter you are in BB6.
4-The invulnerability zone.
That leads me to talk about the "invulnerability zone", because AP shells can be bounced too and that's can be fustrating for some.
There as range of angles where a armored ship can bounce AP and HE shells and good players of armored ships know it perfectly. Some people say that zone is around 15-20 degrees, personally i would say it's around 20-25, well, it depends on the ship, the armor and the shells used.
For example in a example a guam (US BB1) against an armored QV the invulnerability zone starts at 30 degrees, below that angle AP impacts on deck are bounced.
The point is not where it is, the point is you need to know it exists. If you are in that zone you can do 2 things go more near to the enemy and use HE or try to put distance and use AP.
Here is a pic, this are the options (for example, as a reference) about a yammy/superyammy shooting against a AW QV, or against a AW L2.