“and when to hold fast on the boundaries.”
You should ALWAYS hold fast on the boundaries. At least on the boundaries, if not more than just borderline.
And I agree with the many above posters who said we husbands and fathers must require our wives and daughters to keeping the relevant halachas. The above posters are correct, we are equally culpable when our wives and daughters go out of line, and we must insure it does NOT happen.
And no one said we should keep tznius to the exclusion of any other halachas (i.e. L”H, etc.) We should maintain all halachas, including tznius.