Our future building owner is considering to install stormwater collection tank. Therefore we're trying to recommend the owner to comply with the LEED requirement.
As this site's previous imperviousness is greater than 50%, the option 2 would be selected.
According to my calculation, 25% of volume to be reduced is around 600 m3/day.
Is there a calculation method of storm water tank capacity to meet this credit?

In addition, I have another quick question regarding 2 year-24 hour design storm.
I understand that this number is from regional statistical data.
I got the following number from the Meteorological Administration Office;
"In 2007 and 2008, the greatest precipitation of this region during 24 hour was 1355 mm in June."
Therefore I used 1355 mm/day as a 2 year-24 hour design storm to calculate the runoff volume and 25% stormwater volume to be reduced.
Is this correct?