It shouldn't matter. The point of this control system is to make the cars mixture be an exact amount. It compensates up to its capabilities for any change you make. The only exception to this is that all mixture in non steady state is made from base mixture at the correction of the moment.
The point is was your car in CLOSED LOOP. You state 50% and -2.3ma. These numbers beter be moving. A fixed 50% duty cycle indicates an open O2 sensor circuit (or no response - such as when cold). The EHA current should be at 0 when the duty cycle is at 50%. I'm hoping that the 50% was the high point of a 5% swing as this would go with a momentary -2.3 ma fuel correction
The EHA current should be also swinging up and down a few ma.
It is this correction lean to rich to lean to rich that is CLOSED LOOP control. If it is happening you shouldn't fluck the test. Making sure the cat is hot is the rest of the story.
Bosch Master, ASE Master, L1
33 years MB technician