The O2 readings of low .1-.2v to high .7 to .8v are what you want to see and will occur at any EHA reading that is in closed loop. One must understand the concept of closed loop. Closed loop infers control. Control is accomplished by going rich when reading lean and then turning around and going lean once rich is recognised. Where ever the mechanical mixture is set the system will move back to lambda by using the current correction. If it takes 4ma then it will hover about that. If it takes 8ma then it hovers about that. It should not change much once everything stabilizes and the O2 sensor cleans itself.
The way to judge the appropriate average EHA current is to disconnect the O2 sensor and view the current. All six cylinder models centered on zero with plus and minus current capabilities. The the early 4cyl models only corrected with current in the positive direction and in order to have both lean and rich correction they needed to start above zero. Thus 8ma is right for them. If one disconnects th O2 then the current will be either zero or 8ma at idle.
Bosch Master, ASE Master, L1
33 years MB technician