Looking at the picture Guest posted I wonder if the plating around the lower row of air ports is actually armor? Or is it just an ordinary hull plating strake?
The armor belt probably didn't extend above the armored deck (usually not the main deck). So which deck had the thickest armor plate? That likely marks the top of the armor belt.
Also, armor was designed to be effective against enemy gunfire, and actually was of little use against torpedoes. It was an example of preparing to fight the previous war. The armor belts were mostly below the waterline only a short distance - enough that water would slow a round sufficiently that it would lose penetrating power. Torpedoes often hit the hull below the armor belt. The armor was angled to cause the round to ricochet or to break off the hard nose piece, preventing it from penetrating the armor.
The blisters were often added later when navies realized the danger of torpedoes. They may have extended around the armor belt, as shown in your drawing. The blister surface was to detonate torpedoes and the void between the blister surface and the interior hull served to absorb the force of torpedo detonation before it reached the main hull plating. It would do the same for a plunging projectile that bounced off the armor plate.
Here is a web site devoted to analysis of naval weapopns and armor:
http://www.navweaps.com/index_nathan/index_nathan.htmThere are discussions of the "best" battleship design, with descriptions of the armor. You might find something useful here.