During a flight yesterday I tested the minimum MAP required to maintain pressurization and came up with the following (@2400 rpm):

25" MAP = 5.5 psi
23.5" MAP = 5.0 psi

25" @ 2400 rpm equates to 65% power.  Thus, when descending with full pressurization, I keep the power setting at or above 65% power.

The inner and outer rings on my Dukes controller seem to be calibrated for at 5.0 psi differential.  To reach the 5.5 psi design spec of the plane, I set the Dukes 2500 feet under my target altitude.  This consistently gives me 5.5 psi.  

Pete