Choosing belt and deck armor

What belt-to-deck armor ratio is reasonable for a dreadnought-era design?

It depends on the expected fight, but long-range designs need substantially more deck armor than close-range concepts.

Mission and expected engagement range matter more than a universal armour ratio. Long-range fighting increases the importance of plunging fire, so a ship designed to stand off needs meaningful horizontal protection. As a reference point, the Second World War King George V class paired a 373-millimetre main belt with roughly 124 to 149 millimetres of deck armour. That example suggests deck protection of roughly thirty to forty percent of belt thickness for a long-range design. The estimate assumes strong internal bulkheads, which on the same class reached roughly 254 to 305 millimetres. Belt, deck, and bulkheads form one protection system rather than three independent numbers.