Your source for 20% says that 20% is a "conventional estimate" and doesn't seem to provide a source for that estimate. It seems pretty high compared to other numbers I've seen.
Acknowledged - I didn't shop around carefully for primary sources for my link. I think the overall point holds even if you drop it to 10%, though -- there's probably 1% of overall water use to be trimmed through the more-effective residential cutbacks.
(It's worth distinguishing between potable and non-potable water in these analyses, though, because from a municipal perspective, there are additional benefits to keeping a cap on potable water use.)
