Ok. Seriously. This has to stop. Yes, there are halachos concerning tznius but be a bit realistic for a second. The truth is that tznius depends on the person and how they feel on the inside. If a person feels modest, if a person feels that they are above parading themselves before the world as a physical object, then they wilkl naturally dress more modestly. If not, if they feel that they have nothing but their body and their looks and beauty, then they will find a way to make even the longest skirt incredibly provocative.
This isn’t just true of jews…as shocking as this may seem, modesty isn’t an exclusively Jewish or even religious concept. Take a look around you, and you’ll begin to notice. If you have to ask how long your skirt HAS to be, then you either don’t care, or have something else to work on before the question even becomes relevant.