Actually any specific reading 50%, 60%, 90%, 10% etc. indicates a specific fault code.
To be sure that you are in sync, try key on, engine off. The reading should be 70% if a federal version or 85% if its a CA version. Once your are in tune, the constant reading of any multiple of 10 is a specific fault code. A proper running engine in closed loop will not be steady but an oscillation of a few percent about some value. The value to which it centers about indicates the relative mixture. MB would have the final setting oscillate about 50%. A steady reading of 50% indicates an inactive O2 sensor (fault code). I usually set mine at 40-45% as this puts the system slightly rich for everything not at steady state (and cold running).
The EHA ma should be about 20ma KOEO (key on engine off). When in closed loop at MB setting (oscillating at 50%) the ma should also rise and fall about 0ma. Lambda control can move 10-12ma in either direction (- or +) for correction. On decel you will see -60ma fuel removal. A quick acceleration wil give 6-10ma of correction from where ever you started. On cold start the combination of cold enrichment and starting enrichment can be +50-60ma. The key when warm is to observe closed loop operation (cycling between rich and lean to achieve lambda).
There is more about this in the article in the DIY section called Evaluating engine controls.
Bosch Master, ASE Master, L1
33 years MB technician