Maneuver and durability are data file values that are used by the code engine to calculate certain things. It all depends on what code you use. I will only speak to AE, because I had much to do with development of these values in the context of the AE algorithm, and only the AE algorithm.
In so far as durability is concerned, it is a function of an AE value called “tonnage”. “Tonnage” is a functional parameter that has nothing to do with GRT; it is a function of a geometric coefficient x cubic size x displacement.
We chose warship “standard” tonnage as the calculation basis, because it is a physical displacement parameter, and we could get good results off of that for non-Navalized ship vectors. There’s a lot of internal code that tweaks things, and there’s lots of traps for the unwary, so the suggestions are for nominal ships, only.
In AE, figure on 1 point of durability for each 250 tons of standard displacement for a warship. Figure on 1 point of durability for each 500 tons of “tonnage” for a merchie. And figure on 1 point of durability for each 350 tons of “tonnage” of an auxiliary or Navalized merchie.