water use reduction calculation

Structuring the baseline is somewhat subjective, and is largely dependent on the "average" circumstances in your locality. Consider what would be the conventional or typical development strategy in your area. Using a sububan office park as an example, an office facility in such a park in my region would normally have about a 70/20/10 landscape split: 70% turf, 20% mixed (shrubs, perennials, ground cover, etc), and 10% tree. Using the above metrics, if I have a 100,000sf of vegetated landscape, then I would define my baseline case as 70,000sf of turf, 20,000sf of mixed vegetation, and 10,000sf of trees. Your scenario may not be that cut and dry, but you don't need to overthink the baseline. Keep it simple; just make sure you can justify the numbers, and they make sense for your bioregion.