One thing, I don't own a "Z", but my system is pretty much the same, it's uses a "Bosch L Jetronic" system. all I'm doing is changing the resistance the ECU reads from the temp sensor.
The rotary switch is wired for 3 settings, normal, ie direct connection to the ecu to temp sensor, rich - ecu through a resistors to the temp sensor, and lean, ecu through a resistors to ecu earth (no temp sensor).
To work out the resistance, all I did was for lean was, drive the car for 1/2 an hour or so, make sure it was very hot, (hotter than normal driving) ie hill driving, and record the resistance (L1) of the temp sensor,
for rich - start the car cold, leave it idling for a minute or so and record the resistance (R1).
You need to also record the resistance when the engine is in its normal running temperature (N1)
I done this a year or 2 before I got my wide band, so doing the above I though it should be with in a safe limit
on my car the lean setting is about 1 and a bit AFR leaner than normal, I can't select this setting until the engine has been running for a few minutes, else it has a nasty dead spot just off idle.
I've included a picture that may help
The resistance for rich setting would be "N1-R1" ,
the resistance for the lean would be "L1"
I hope this makes sense
Nigel