At what point is it more efficient to build ships than harbors?

With the cargo ships upgrade, every ship gives a 2% bonus to harbor capacity. I've noticed that my harbors cost ~290 scaffold, ~2800 slab, and ~4000 plate, which with the 105 ships I have give an additional 1437 wood storage.

On the other hand, building 3.94 ships provides about 8k wood storage for 100 scaffold, 150 plate, and 25 starcharts.

Clearly, I've built too many harbors as it is far more efficient to build ships at this point.

What is the point at which building ships becomes more efficient than building harbors? Does this at some point get hit with diminishing returns (which would then complicate the equation) or is it once you've hit that point, its always better to build ships?

For ease of calculation, the efficiency should probably be restricted to scaffold to make the comparisons easier unless the other materials become a much more significant factor.