Why modern armour does not yield a simple improvement figure for Hood

How much would HMS Hood improve if her First World War armour were replaced with newer plate?

It cannot be quantified without a protection scheme; designers could use equal thickness for stronger local protection or thinner plate to cover more area at the same weight.

The benefit cannot be reduced to one percentage because the protection scheme matters as much as the improved material. Replacing old plate with equal-thickness modern armour would maximize resistance where Hood was already protected. Using thinner plate could extend protection over more of the hull without increasing displacement. A later exchange asks whether the five- and seven-inch belts should be removed, which depends on the threats those belts still defeat. A coherent distribution could add protection over machinery and magazines instead of trading every saved ton automatically for a thicker weather deck.