Journeyman wrote:

It may not be linear but it is monotonic. Increase the MP (at a fixed
RPM) and you get increased power.

Very indirectly! Change the angle of the throttle plate, thereby causing more
or less air pressure drop across the restriction, thereby causing the MP gauge
to see a different absolute pressure.

Change the rpm, and the volume of air flow past the throttle plate changes,
thereby causing the MP gauge to see a different absolute pressure.

Take the airplane to a higher altitude, thereby causing the MP gauge to see a
different absolute pressure.

Let the air induction filter get dirty, causing a flow restriction, thereby
causing the MP gauge to see a different absolute pressure.

Let the temperature/humidity change, thereby causing a change in air density,
thereby causing the MP gauge to see a different absolute pressure.

What you really want is a Mass Flow sensor. Every modern automobile has one,
but our 1930's technology aircraft dont...

MikeM