Sounds like a discussion that I think B3 published, where the 2/3 lift point was used for the idea of minimizing scrub forces on the valve with old non-roller type rockers. Like 80 years ago!
The idea was that at the top of the lift cycle, the spring forces are their highest, so to compensate for that, then the sideways motion in the upper part of the rocker motion is made less, and the resulting net side force on the valve (due to the friction of the rocker tip scrubbing across the valve) was made less. The largest scrubbing motion was then moved to the bottom of the lift cycle, where the lower spring forces resulted in less net friction related side force on the valve.