I don't know that there is an actual level for slide operation. What is normally considered the determining factor is that you have the stabilizers down to prevent frame twisting prior to operating the slide.
Jake my sidekick (yellow Lab)
2017 RAM 2500 CC 4X4 Cummins Diesel
2016 Flagstaff 26 FKWS
AF&AM & El Korah Shrine of Idaho