Why lower FAC partial changes?
Lowering the tenor partial change to G#2/A2 results in a full 2-octave (A2-A4) range with all notes within that range using their 4th partials for the tuning. Not having any partial changes within the A2-A4 range, makes the tuning more accurate by eliminating even the possibility of a partial change error between A2-A4. In the treble, using the 1st partial instead of the 2nd @ A5 makes determining and tweaking the treble stretch easier.