Cistern volume

Could you please give me an advice regarding question 1? Our cistern is designed to store the volume that is required in this credit – the predevelopmet runoff is 90 m3, the post development runoff is 130 m3 and the difference 40 m3 we would like to store in the cistern (60 m3). The captured rainwater is reused for flushing – the need of water for flushing is 4 m3 per day that means, we could empty the cistern in 10 days after design storm. I think this assumption is not sufficient in order to meet the credit requirements. We must declare that we reuse all the captured rainwater as states upwards. Is it sufficient to calculate annual balance of rainwater that is captured by the cistern and the amount of water that is reused for flushing in monthly steps? Which time step do you recommend? Another question: which part of the monthly rainwater volume I must reuse for flushing to meet the requirement of SS6.1? I think it is not necessary to reuse all the rainwater because the credit requires retaining the predevelopment conditions (90 m3 during design storm). Is it for example possible to reuse 30% of the monthly rainwater volume to meet the content of this credit (30 % results from the design storm – we must capture 30 % of rainwater = 40m3/130 m3 to meet the requirement). Could you describe briefly how it is possible to declare sufficient volume of the cistern and water for reuse? Thanks a lot!