The standard already exists
In criminal and regulatory contexts, evidence that cannot be traced to a preserved original is treated as unreliable. Financial analysis is routinely accepted with no equivalent discipline.
A number in a board pack usually cannot be followed back to the record it came from. If the number is later challenged, there is nothing to defend it with.
What preservation requires
Take the original exactly as received. Record who provided it, when, and in what condition. Compute an integrity value on receipt and never work on the original again.
Every later step, including every correction, is appended rather than overwritten. Nothing is deleted, because deletion is itself a fact worth recording.
PERISCOPE briefings describe investigative principles. They do not disclose detection logic, thresholds, formulas, weighting or rules.