it will depend on how much water is in your sump, etc.
I set mine so the bubbles are ~1" into the cone part of the collection cup, then the skim will start wokring itself up the cone and over the top.
No specific setting, just need to adjust it until its coming over the top, you will need to adjust as evaporation ocurrs.
If its brand new it will take a couple of days to break-in.
Follow the directions, read them 1st then setup the skimmer.
Also important to have the pump ~3" below water line.