Calculating stored rainwater available

We are having some trouble coming up with the calculation for rainwater stored and reused annually. We have a roof area of about 700 Sqm with an annual rainfall of 700 mm and a storage capacity of 50000 litres. A lot of this rain happens in a short span of time say two rainy months. Most of the other months are quite dry. We are absolutely certain that we have sufficient capacity to store two days of peak rainfall. But we are not certain that we can use the entire capacity quickly enough to make room for more rain. Specially if it rains during weekends. I was wondering is there a calculator that can help us come up with a LEED accepted annual rainwater reuse for our project? Many Thanks Yusuf