Reducing Volume of stormwater runoff through 2 improvements

Hello, Our project is based on the case 2. Existing Site With imperviousness is greater than 50%. We implemented several strategies for improvement: a) Reduced runoff place b) Accumulation of rainwater for reuse in toilet flushing We calculate reducing stormwater runoff by reducing runoff coefficients. The result is an improvement of 12%: Pre-development = 508,5 cf Post-development = 447,2 cf Then, we calculate the water reuse for toilet flushing in 72 hours. The result is 500 cf. (The tank capacity is 1550 cf) So, can we prove that our post-development runoff is 0 cf (447,2 cf - 500 cf)? Thanks