According to my own (unscientific, unpublished) study, there is a large variation (taken over individuals) of the exact position where the right hand pinky hit the Shift key on the standard keyboard. Somebody hits the left edge while somebody else hits the middle of the key. People tend to hit the gap between it and the upper key, i.e, the Enter.
(The right edge of the right Shift key on the standard keyboard is also hit frequently, but by the index finger in navigation+editation mode. So it does not apply in case of mini boards.)
Therefore, if we can afford column-staggered layout in a design for the mass, the 1.5u width and 0.5u vertical stagger for the outermost column, like the Ergodox, is pretty reasonable.