Training says that 50% is the proper spot. Training will also say that it doesn't matter as I have pointed out.
BUT, -2.3 is the way I set them up. They still have plenty of run for correction although it is no longer 50/50. I prefer basic fuel correction a little richer for some performance reasons. Remember electronically everything is made to be the same mixture, but it doesn't happen instantly. It takes a half second or more. For instantaneous throttle response I like the basic mix slighly richer. If you run around jumping on and off the gas the thing might not stay in emissions control.
It also is a little richer cold, as electronic control is only woking in fixed mode (no lambda correction til O2 sensor is hot).
It is also a very small amount.
Bosch Master, ASE Master, L1
33 years MB technician