You might find more information on EHA theory of operation and adjustment by searching for the appropriate Bosch Jetronic technical information (K, KE).

It is my understanding that the EHA valve adjusts the engine for more rich or more lean based on information provided by the oxygen sensor.

And that a new EHA should be adjusted about right? That adjustment being in the "center" between rich or lean.

So the engine computer could send a + - electrical signal to the EHA and it would adjust more rich and more more rich if more current is sent. And the opposite, - + and it would adjust more lean.

Sort of like playing tennis. You hold the tennis racket out in front of you in the center. Then you are ready to swing to the left or to the right should the ball go either direction.