> Just a fairly arbitrary number. It's easy to produce a few lines from memory, up to 10s of lines and that's "obviously" fair use.

it's anything but obvious. https://www.copyright.gov/fair-use/

> there is no formula to ensure that a predetermined percentage or amount of a work—or specific number of words, lines, pages, copies—may be used without permission.

9 lines of very run-of-the-mill code in Oracle / Google weren't considered fair use.