(e.g. four inches)
That isn’t a geder. You have to cover the knee. However, the knee must be covered even when sitting, walking, and getting into/out of a car as well, and if your skirt just covers your knee while standing straight it will not cover it in other positions. Therefore, some length has to be added. There is however, no Halachic quantification for this. If you need 4 inches, then it’s 4 inches. If you need 3, 6, or 8, then that’s what it is.
Completely agree. That’s what makes “four inches” so dangerous. Four inches is what they teach in the Bais Yaakovs & Sems (from what I hear), and it really depends on the person (& Skirt?). If they just taught “cover the knee in all situations” that would be good and Halacha (according to most, not getting into that here). Making up a Shiur is being M’Garaya from the Halacha.
An additional example (to get out of Tznius) would be having two sinks, and how a sem tought a friend’s daughter that if she does not have two sinks, she is eating Tarfus.
First of all, you are unfortunately and tragically incorrect, the divorce rate among the Yeshivish has become increasingly higher in recent years. I myself know at least half a dozen couples who broke up within the year of marriage.
That is because we Frum Yidden are concentrating more on externals (e.g. type of tablecloth) vs. internal (e.g. Yiras Shmayim). If Your children are able to shut out the Shtusim and concentrate on the Pennimius, they will Bezras Hashem have a good marriage.