The specific rules usually quoted (counting letters, finding the middle word, discarding a scroll for one error) come from later Talmudic and post-Talmudic sources, and the most colorful versions are exaggerated in popular apologetics.

The rules do need dates attached. The Masoretes compiled letter and word counts and noted the middle letters of books, and those counts survive in the margins of medieval manuscripts, so the core of the claim is documented. The rule that a whole scroll is discarded for a single error is a Talmudic-era standard for Torah scrolls and should be described that way, not as a general practice from the beginning. These rules explain the stability of the medieval text; the earlier stretch from the authors to Qumran is known only from the manuscripts themselves, which show careful copying alongside real variation between text types.