Now I'm curious. If I were to design a modern version of a Nesbit S.G.'s skirt, I'd go with fitted and curvy, with back kick-pleats releasing from the knee down. To evoke a slight Edwardian train, maybe I'd have the front knee-length with the back pleats just a little longer.

But I wouldn't call that "mod" at all, so your platonic ideal of a Nesbit skirt is probably different.