I understand that one set screw allows extra air into the IAC and the other allows it out to the motor at idle but how am I to know which I should adjust to say solve an issue with the car not getting enough air on return to idle after a high rpm pull? Would I allow more air into motor or more into IAC or both ?
Unfortunately every case is unique and it is tough to answer that. You will likely have to adjust both to reach a happy medium. Before purchasing this though, I'd recommend ensuring the IAC is functioning properly is not sticking or a tired actuator as this will simply be a band-aid fix if that is the case.