Berto’s 15% rule was based on the crappy clinchers available at the time, which did degrade performance at lower pressures. It was done with actual width, not the lies printed on the tire. Rim width is both a small variable, and easily accounted for by measuring the net width. And, it is just a starting point so you don’t go crazy hard or soft for a given tire size.

There are still many crappy tires on the market, especially wider ones, but good tires don’t change rolling resistance at any useable pressure. Personally I shoot for 20% sag as a starting point and have set all my PB at this level. If you don’t get a pinch flat every now and then, you probably are over inflating.