I am going through the same learning experience as you are due to idle/hard start issues on my 91 560SEL. As I understand, the on/off ratio and possible problems (per the manual) you mentioned above are true with the following conditions; that the car is in operating temp when on/off ratio is measured; that the reading is constant, meaning a reading of 50% for example does not fluctuate, then it incdiates an O2 sensor problem. You can try and simulate a fault condition, and measure the on/off ratio, and compare it with the table you mentioned above. There is a concept called "closed loop" in the injection system which says that the on/off ratio under certain conditions will display a fluctuating reading ( ie 40% to 60% for example). This fluctuation in readings indicate that the O2 sensor is continously monitoring the exhaust gas and giving a feedback to the CIS control. The CIS control module in turn sends a signal to the EHA that then regulates the chamber pressures in the FD. The on/off ratio is a way to measure this normal behaviour as explained in the articles you mentioned above.

Others may have more informed inputs regarding this topic...