Establishing a Baseline

You might have to cast a slightly wider net in terms of recruiting some expert help, as this is a tough thing to get reliable figures on. Perhaps a landscape architect could help you identify a rough sketch of conventional landscaping for the region, and then your landscaper could help estimate how much water per SF they would recommend for that type of landscaping? You might also touch base with local water conservation and/or xeriscaping-type organizations. There are a number in the South Bay - the Central Coast Graywater Alliance comes to mind as a starting point that might help you get there eventually. Hope that helps at least a tiny bit.