Depending on the nation. I like to use .4 with US/KM or .5 with IJN.

OR... You can ditch the bulge and run belt. Belt over 10 inches will usually stop almost
any shell that's fired from long range but it MUST hit your belt. It's a rare chance but I've
found it handy sometimes. It will also bounce bombs that hit belt as well. It can also
reduce splash damage from shells.

so if u have more than 0.2" of bulge it's okay too, or is .2" the optimum amount of
armor?

What you have on your ship is pointless. 2" of everything is about as useful as 0" of
everything in stopping shells.

Most ships cannot armor effectively so unless you have 10~12" of deck your ship will
be as effective as having none at all. Armor in this game is either all or nothing so it
will either stop ("bounce") shells or absorb it even if you have say 5".

Here are some pointers
-0.2" belt as stated above halves torp damage
-For UK ships 0.2" deck, 0.2" belt, 2 ton bulk is the magic number that rumors to
prevent crew death.
-Shells fired by enemies less than 30 degrees hit belt, and over 40 (i think) hits deck.
In between is the immunity zone. So if you play close range in blitz, load on belt. This
is why BB mostly use deck and therefore the QV 30 degree guns are so deadly.*
-General rule for blitz is belt thickness stops same diameter shell (3" belt stops 3"
shell almost)
-IJN cant armor for beans

*Need verification on on immunity zone

0.2" belt + bulge without losing any knt of speed in GB/GB2
speed > armor
but if you play blitzkrieg you might want to armor your ship, use deck if you can stay at
range, or go full belt if you like to do close quarter combat.

in any case, keep 0.2" belt, so torpedoes will do only half damage, never go under 0.2".