I haven't been able to read the opinion yet, as I'm on a phone, but often these types of thresholds or tests come from related cases that have been previously d

I haven't been able to read the opinion yet, as I'm on a phone, but often these types of thresholds or tests come from related cases that have been previously decided. Natural places to start looking in this case would be the Quill Corp. v. North Dakota or National Bellas Hess Inc. v. Illinois Department of Revenue. (If not there, following the footnotes in Kennedy's opinion will likely get you there eventually.)