As Steve said, you want to see 85% key on engine off on CA models. If you read 15% and can't reverse the polarity of your meter, just swap the leads.
As the Landiss write-up says, there is no actual spec for duty cycle, but if it's in the range of say 40-60 percent at both idle and 2000 RPM, no load, it's okay.
If the duty cycle was at exactly 50% it would mean that the basic mechanical mixture adjustement is right on stoichiometric, but it's imprecise, relative to the O2 sensor, electronics, and EHA correction, so the actual duty cycle will usually vary in a range of 5 - 10 percent. If it's near 50 percent then the system is operating near the center of it's operating band. You should only have to adjust the mechanical mixture adjustment if it's operating near the edge of the band, which would be indicated by a very low, say less than 25 percent, or very high, say over 75 percent duty cycle.
When I tested my 190E 2.6 last spring I had a duty cycle in the range of 40-45 percent at idle and 50-55 percent at 2000 no load, readings which I deemed acceptable. Since I also had a scope I pulled the O2 sensor signal lead out far enough to connect a test probe so I could eavesdrop on the O2 sensor voltage signal and the waveform was nominal.