Fair warning, I just recently mucked with this same screw on my 380SL and found myself in a world of hurt.
I highly recommend you borrow an oscilloscope and use that to set the screw by reading the waveform that comes out of pin 3 on the diagnostic connector. I know it's verboten to link to other forums but this guide here will help you figure out how to properly adjust the screw using an o-scope:
K-Jetronic WITH Lambda Overview and Adjustment - Mercedes-Benz Forum
The link redirects to BW. In case the redirect goes dead, simply google K-Jetronic Lambda Overview with Adjustment.