I have a question for a Czech colleague of mine. The have a project and want to install a stormwater storage tank. The water shall be used for toilet flushing in the building. The problem is, that he does not know how to determine the design storm interval. I looked into the LEED BD+C Reference Guide on page 97 in the example (last calculation) at the top of the page. From where do the 3 days / 72 hrs come. I understand that the tank must be emptied, but I cannot help him identifying the design storm interval. Can anybody please help?