The highest uncovered hole bit is what threw me off. Where that gets into trouble is with forked or "cross" fingerings, which my NAF mode has a lot of. For example, take the normal octave note where just "L3" is down: 00X0000. The highest uncovered hole is "L1" and that controls the slide action. My intuition was trying to tell me to use "R1". I'm not sure what to call that hole - "hole just below the lowest covered hole"? (Not exclusive to the NAF fingering though. Notes like the C natural whistle note 0XX000 behave the same.)The way slide works is that lowering the finger over the highest uncovered hole will gradually flatten the current note down to the next lower note on the scale
Of course, changing the slide hole would affect which holes are available for vibrato.