But what if for some reason your eye started at her blouse (maybe it's really cute or a really flamboyant pattern or something). Isn't it possible that you'd then say "down"? I would.I don't usually disagree with Kate, but in this case, I do. If the slit starts at the hem (what you call the "low edge"), I'd definite use only up, not "down".
I seeIf I am pointing out how much leg it being shown with the slit, then I would say it is "slit up" (or "slit up to her thigh")
If I am pointing out that the pencil skirt allows freedom of leg motion to a greater degree that other pencil skirts then I might say it is "slit down".