See, that's the puzzling thing - my maths must be off, but, hull length, 165m, bow is 25%, quarterdeck is 20% (NSWE ships have to be very seaworthy, Cape Horn is right there...), leaving 55% for citadel ... 55% of 165m is ... 90.75m, the belt length. Have I missed something really obvious?
But do your Mags and ER/BR fit inside that citadel !
It may be just telling you that everything doesn't fit inside that citadel, rather than it not being as big as you think it is.....
I think normal length is a fixed % of water line length (so you don't sink with flooded ends)
The warning that it doesn't fit is volume based as making the TDS thicker makes you need a longer citadel (it gives you the length needed at top of the armour page on my version of spring sharp)
Oh, also, deck armour - it's as thick as it is because those were the figures I could find for Nassau - does seem a little excessive, though, given the era, doesn't it?
It may be the average of a thinner deck and thicker sloping edge parts ?