wrote:
The old no-more-than-square thing was a rule of thumb for pilots
who flew engines that had little or no operating instructions,

Keep in mind that there's nothing magic about "square" operation. Square
means "the manifold pressure is that same as the prop speed". That's
hogwash; the numbers only work out the same because of an accident of what
units we use.

There's nothing that says we have to measure prop speed in RPM; we could
just as easily measure it in radians per second or Mega-degrees per
fortnight. There's also nothing that says we need to measure manifold
pressure in inches of mercury. It could be in mm/Hg, torr, atmospheres,
PSI, Pascals, etc.