[NTG-context] vertical kerning for TwoThird needed

Hi, comparable to the pre-defined \threequarter I want to write a \TwoThird. The following looks quite good (with my respective font settings) ... \def\TwoThird{{\tx\high{\txx 2}}\kern-0.1em/\kern-0.1em{\txx 3}} ... but still the "2" is a bit to high. How can I adjust "vertikal kerning"? Thanks, Steffen