It depends on many factors, the first of them being what ship are you hitting when dealing those values. You will deal less damage to higher tier ships in general.

Setting that aside and assuming you are hitting the exact same ship different times (just as an example):

- For close range/low angle shots, the defense is Belt.
- For long range/high angle shots, the defense is Deck.
- For Torps, the defense is Bulge.
- To maintain your Overheat capabilities at low DP (when you have received a lot of damage), you use Bulk. 

There is, of course, a LOT more to it.  So let me tack on some common useful advice.

 - The dividing line between hitting deck armor and hitting belt armor is 20 degrees.  If youre firing your guns at over 20 degrees, youre more likely to hit deck, and vice versa.  If you're firing right AT 20 degrees, your shot might hit NEITHER and skip off doing little to no damage.

 - Against gunfire, deck armor is pretty much all or nothing.  It either stops the round cold or it doesn't.  It is only worth mounting deck armor on your ship if it can hold enough to stop common sizes of incoming rounds.  Check in the fleet sections of the board for setup guides to see if you should even bother with deck armor at all.  Most often you'll either run with none or with a token 0.2" of it.

 - Belt armor works much the same way, and belt armor is lighter, so you can have more of it.  But no ship has enough spare weight to pack on both belt and bulge.  You'll see high amounts of belt armor only in blitzkreig games where death often comes from short range.  In higher level play, death is dealt out by battleships at longer ranges, so belt armor is hardly used.

 - The exception to that is torpedo damage.  While BULGE armor is what soaks up torpedo damage, BELT armor knocks a percentage off the damage that torpedoes and mines inflict on you.  0.2" of belt will nearly half the damage that you take from such an explosion, and the reduction tops out at about 70% with 1.5 inches of belt (but that's a lot of extra weight for very little effect).  So common wisdom says to put about 0.2" of belt armor on your ship and THEN bulge armor until you're about to slow down from the weight.

Another point that has yet to be addressed is Soft Defense (SD). A ship has a cap of 900 SD which gives about a 90% reduction to flat out damage and puts it instead into "burning damage." This is the blue arrow on the health bar ingame. Most upper tier BBs (4+, and some EBB3s depending on crew) will have 900 SD and are therefore much harder to kill.

The armor is measured in inches, so 0.2" is the same as 0.2, yes.  We recommend that amount if you plan to face down torpedoes.

And the 1.7 deck armor isn't doing you much good at all, it's just heavy.  Actually it is stopping 3" and maybe 4" shells from long range, plus reducing the damage you take from dive bombers.  But the things that are killing you most often are either up close or they're bigger than 4" shells, and your deck armor basically doesn't help at all against that.  You would actually be MORE survivable to strip all that armor off so your ship goes faster, or convert it into bulge armor to weather torpedo attacks better.

You'll need FAR more than 1.7 deck to bounce incoming 8" shells, and you likely cannot do it with your current ship.  Since you're running IJN ships, the short answer is that there is no IJN ship that can mount efficient deck armor to stop incoming shells.

Check the IJN forum section for specific ship setup advice.

1.7 of deck will probably not stop much of anything...especially with IJN. UK is the best known nation for AWing, although I have seen some pretty effective Alsaces with the rear turret removed. 11'' of UK deck will stop most BB shells in the game. 1.7'' will not be near enough except perhaps in a blitz level. Although IJN armor is very weak and I wouldn't be surprised if it didn't stop anything at all.

On the question of the 0.2": The - in this case means absolutely nothing. He means simply 0.2" of belt armor will halve torpedo damage.

Wow is IJN Armor that bad?!

So did I make a mistake choosing IJN as my nation?
I'm also disspaointed of the shells damage of IJN...
The range is good but the damage is pretty low. 

Wow is IJN Armor that bad?!
So did I make a mistake choosing IJN as my nation?
I'm also disspaointed of the shells damage of IJN...
The range is good but the damage is pretty low.