I try for just enough above the knee so that some narrowness shows there. Man this is hard to explain ... if I cut myself off mid-calf I look like my legs are that heavy all the way up. If I reveal some of the definition, that is the knee joint, one could assume that my legs are that slim. If I go any higher, you can see that my thigh widens ... and could be huge under there.
And there's the business of needing to be able to sit down
A straight skirt? mid knee cap. A flared skirt is a bit more forgiving and can be a little shorter.