DaasYochid –
Das yehudis is not a standard which we are told to make, it is a standard based on observation. The mishnah (in Kesubos) is saying that the Jewish women happen to have a minhag tznius which is stricter than the non-Jews, and therefore the agreement to live together as a Jewish couple is an implicit agreement to live according to that minhag tznius. What if there is no specific minhag tznius, are we supposed to make specific rules based on the old minhag tznius? There is no indication to support this. If there is a apecific minhag tznius in one locale, does that bind another local which does not have that minhag? There is no indication to support this.
I’m all for a minhag tznius, and by extension, a halacha of das yehudis. But let the community decide its minhag tznius for itself. As for halacha, I don’t think there is any real source that any particular body part is obligated to be covered regardless of the society one lives in.
If I were starting a community I would say to look around at the general populace. When you dress up, don’t dress the way they dress to kill. Be a bit more low profile than that. That is a minhag tznius which is a higher standard than the general society around us, but at the same time does not bind everyone from the beginning of time until the end of time to the same arbitrary, absolute standard. I don’t think Chazal really said anything more than this.