What do you mean, "the many women who go to SSPX chapels in pants or in skirts that don't even reach the knees."
That isn't my experience at my Texas SSPX chapel. Texas is a hot place, too.
Once in a while, we'll have ONE woman in pants -- always a visitor. That's it.
In defense of the OP, there was an OR in there and while I only see guests wearing pants to Mass, I do see a good number of too-short dresses.
I would say there are a number of women "riding the line" or trying to see what they can get away with -- wearing knee-length "while standing up" skirts/dresses.
But isn't that the minimum standard?
I think the standard should be "higher" (pardon the pun): ankle-length.
Of course, once you get stricter than knee-length, it's time to buy a sewing machine. Not every woman is that generous (to souls, Our Lord, etc.)
Some want a measure of convenience, acceptance by the world, etc. For them, knee-length is where those two standards meet.
While I strongly disagree with these women, I would place them in a different category than "above the knee"-skirt-wearing women, for the sake of accuracy if nothing else.