When using the same input data in both forms (IP and SI units) someone should expect the same result (percentage mitigated). The online form with SI units gives a negative percentage mitigated and the quantities of "Total rain from design storm" and "Total runoff from design storm" are unrealistic. Especially when the total runoff is greater than the total rain. 

Is anyone else facing the same problem ? Anybody knows if I do something wrong. Where can I find the formulas used in the online form .

Thank you