Regarding tzinus for a woman: Is it covering the thigh to the knee or beyond the knee or the knee as well? Are there shitos that hold the entire leg as well?
The principle halachah is that the thigh must be covered, up to and including the knee. This is the ruling given by the Mishnah Berurah (75:2), and many follow his ruling. In a practical sense, the skirt must several centimeters lower than the knee, to ensure that it is covered at all times.
However, many authorities (including the Chazon Ish 16:8) write that the “shok” of the Gemara is not the thigh, but the leg beneath the knee, reaching down to the ankle.
According to these opinions, the entire leg, down to the ankle, should be covered. However, there is no obligation to cover the entire leg with a skirt or dress, and for the area beneath the kneeit is sufficient to wear tights.