AAQ – young men shouldn’t have to know what to do if their nother is not tznius; it’s a very sad thing indeed and improper chinuch either way….
I don’t think there should be a distinction made between tefilin and tznius; both are discussed by halachikally binding(pardon the pun) literature and both have certain parameters that are completely universal, while tznius also includes standards that are subjective, the poskim make it clear which things are universal and which things aren’t.
Single women covering their hair is subjective. Knees, elbows and collarbones are not. The latter are the things that are most derided, based on the supposed ridiculousness of measuring such things with rulers etc….you don’t hear the same derision regarding issues that depend on local custom.