Need standards or procedures to proceed with the feasibility study of rain water harvesting in Maharashtra

I work for a company located in one of the MIDC (Maharasthra industrial development corporation) sites.

Our monthly water requirements are around 18,000 (m3) and we are considering the possibilty of a rainwater harvesting system. Almost the complete factory area is cemented with all the water from factory floor and rooftop collects in a common drain (this does not include factory contminated water) 

Would a simple (land area) x (average rainfall) measure be sufficient? or do i need to take ome other measure into account?

Do you have refernce to some standard sheets/procedures?

I wanted to asses the net gains before we look into this project more seriously. Any help would be appreciated.

Many thanks,

Manish

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