Your calculations are correct. You will have to develop strategies to infiltrate, evapotranspire, or reuse 260.57 m3 of stormwater to qualify for the credit..
However, depending on your location, please verify that 0.115 m is your 1.5 year, 24 hour storm. That seems high. You are trying to find the rainfall amount for the 1 and 2-year 24 hour storms.