A properly cut nut should have a similar string height above the first fret as a string closed at the first fret has above the second string and so on down the fingerboard. A capo should close all strings to the fret with enough tension that they do not buzz but not enough to stretch them sharp. The curvature of the fingerboard is controlled by the truss rod, and is critical, but but it can only be properly set if the nut and saddle/bridge are without an other underlying problem. The advantage of the Schubb over the Kyser is that a set screw adjustt the tension. However, the Schubb can not be easily clamped on the headstock and it is slower to move to another position. A twelve string capo needs a stronger spring because there are more strings and they are of dis-similar diamiter.