Can anyone recommend a resource to use for sizing a cistern?

By using the LEED Online forms, it is easy to determine the FTE and the daily or annual water use. By using climate data (annual rainfall) and roof catchment area, you can calculate how much rain is available. But how do you determine the appropriate, efficient, and accurate cistern size? If you don't have a large enough cistern, you run the chance of going dry in times of unexpected low precipitation levels, and if you size it too large, it becomes a budget killer and water gets stagnant.

Any suggestions?