[Yoda]

...there are (at least) two ways that the Clubhead can go into Impact with negative loft -- either travelling upwards (after Low Point) or downwards (before Low Point). Which do you think is optimal?

Another useful point to note is that the Clubhead will be travelling outwards before Low Point and inwards after Low Point, even though you can hardly see this given the steep plane used with the putter. Visually and psychologically, this may be a good reason to place the back of the Ball precisely at Low Point.

Any thoughts?

Golf Stroke Geometry does not change with the choice of Club. With the Ball positioned behind Low Point, Impact optimally will be Three-Dimensional, i.e., Down, Out and Forward. Impact occuring after Low Point will result in an Upstroke Motion and Compression Leakage (2-J-2).

For those monitoring the Arc of Approach and its curved Clubhead blur, locating the Ball at Low Point may indeed have visual and psychological implications. For myself, I Trace a Straight Plane Line with my Right Forearm and #3 Pressure Point and am totally oblivious to the actual Arc of the Clubhead through the Ball.