Ismalley. 55% is fine. The closer to 50% it gets is the optimum, but anything between 40-70% seems to have no effect at all from my testing. Several factors will easily affect the reading by 5-10%, temperature being the major contributor. But,,, the numbers at idle and 2500 RPM should be within 20% of each under when at normal operating temp. I didn't test your formula - the actual duty cycle as MB measures it is opposite to normal convention; but if it's 45%, that's fine too. As stated above an inexpensive Sears multi-meter 382139 can be used to accurately read the DC. Just be sure to plug the RED wire in the gnd pin #2 on the round terminal - exactly opposite of what one would think.

DG