Path 2 Stormwater Quantity Control

First of all, please double check your math. 600 cubic meters per day is an awful lot of water to have to store, divert for an alternative use and/or infiltrate. Sizing your storage facility will depend on several factors including how you intend to use the water and the frequency of your rain events. Once you know your daily usage demands I recommend talking with a civil engineer and/or storage tank vendor for sizing assistance. Your approach to determining the 24 hour design storm looks correct, but please note that if this value is higher than the 10 year annual average (i.e. unusually high) you should use the 10 year annual average instead (per page 5 of the 2009 ID&C Reference Guide).