Has anyone ever used "Fuel Trim Setup" using WDS?

I’m looking for some information on how to complete the “Fuel Trim Setup” option under the WDS menu.

When I select it, it asks for CO readings. Where do I take these readings? From the tailpipe? From the O2 sensors, before the cats?

Is there a good default value to use? I’m hoping someone here has some experience with this option in the WDS.

Thanks!