20% not a gimme any more

It is certainly more challenging, but it also can strongly depend on your occupant types. In a hotel project, you'll find that showers dominate water use (at least, the water use that's considered in LEED...). If you've got a lot of students/visitors, waterless urinals have a more significant contribution. 1.5 or 1.6 gpm showerheads are out there and are good. For a typical office building, you might also go to the 1.0 gpf pressure-assist toilets. That makes 3.0 gal/day for female users vs. 3.2 with regular dual flush or 3.8 for commercial dual-flush. But it's still not 40% at the fixture level so you need to make up for it with the urinals. I wonder where the occupant usage rate data originally came from. It seems like this might be an area for future research by USGBC to test and verify these assumptions, since they are the foundation for the indoor water usage estimates.