The original experiments were some time ago, but as I (vaguely) remember Frank Berto first ran trial runs to determine tire drag at various combinations of load, width and pressure. Then found that the most efficient pressures tended to cluster along lines that corresponded to roughly 15% drop.

Please don't hold me to this, it was a while ago. However, similar experiments have been run since, and the general consensus is above a set pressure (related to width and load) rolling resistance on good pavement flattens and doesn't improve meaningfully with more pressure. However the drawbacks of excess pressure begin to be more significant. So it's not like there's a magic number or tire drop.

It's that the best balance of low rolling resistance, and handling properties tends to be in that area.

BTW- it's not like this is new information, except that Frank B. put numbers to what experienced riders managed by rule of thumb. Before people started applying "if more is good, most is best" approach to all things bicycle, from tire pressure, to tire section (narrower), to frame stiffness, & spoke tension, cyclists got help and the benefit of experience passed on from older riders through the club system.

In my cycling career, I've seen fashion trends for extremes in just about every aspect of the sport, only to pull back to "normal" as the next hot thing came along.

Whether you use these tire drop numbers, or your own subjective judgements, or some combination of both, you have to find what works for you, and learn which changes will improve or worsen a property.