If you haven't found it already, Zurn has an online water use calculator that features a LEED option for comparisons. It allows you to quickly check the difference between various efficient fixtures for Water Efficiency Credit 3, Water Use Reduction. It isn't perfect (doesn't show total use reduction percentage), but it's extremely quick. It allows you to pick your scope (ie... if showers aren't in the project you simply don't include them), and the baseline fixtures are already loaded for you!
Zurn Water Calculator
I could explain further, but just check it out yourself! Know of a better resource available for checking water use please share by leaving a comment.