As another poster mentioned this is literally why we have courts. There is a clear line for obtaining a search warrant for example. Precedents exist.
Perhaps you think all FISA rulings should be public and any sufficiently savvy malicious actors can just read them to know exactly how to avoid suspicion?
The FISC process is used when the purpose is foreign intelligence rather than domestic law enforcement, and it exists because prior to that there was no limit on the covered activity when it was done for that purpose.
> Precedent exists, but nobody outside the national security state actually knows what it is.
Well, some of it.
https://www.fisc.uscourts.gov/public-filings
https://law.justia.com/cases/federal/appellate-courts/fiscr/