Will adjusting my string height require me to mess with the truss rod?

[Hopkins]

Actually, the 12th fret harmonic is always in tune with the open string -- it's a law of physics.

Intonation adjustment is a matter of getting the fretted note at the 12th fret in tune with the open string (or the harmonic).

Right, I stated that wrong.

 

[Turnaround]

Actually on a 21 fret neck ten or eleventh fret is right in the middle holding down first and last frets. But......anywhere in the middle will get you close.

Not even close. On a 21 fret neck, the mid point is near the 7th fret. And since there is no flex where the neck attaches to the body (the last four frets or so) the mid neck flex point is nearer to the 6th fret. Unless the trussrod is adjusted at the headstock, in which case it will be back closer to the 7th fret. But that also depends on where the trussrod is anchored and which end it is adjusted from. But on a 21 fret neck, the mid-flex point will not be near the 12th fret.