So if we just made up this number, how is that "data-driven"? Perhaps we need to sit down and rethink this and decide if that's truly the best number.
Sometimes numbers are just not based on the data. This number is from 1991, probably not research-based.
One of the reasons to give homework is to teach responsibility. I heard a good idea the other day for teaching responsibility: make the kids responsible for a clean and well-maintained school, rather than a janitor, like they do in Japan.