We have just realised that the percentile value given by the LEED Rainfall Event Calculator (https://www.usgbc.org/resources/leed-v4-rainfall-events-calculator) was incorrect, just before implementing GID on a project, the actual required volume to be managed is >2.5x than the LEED calculator so we ended up having to drop the credit.

The calculator requires input of daily weather data and calculates the percentile rainfall amounts for you but it doesn't tell you in instructions or comments to remove the rainfall of less than 2.5mm as per the reference guide. We expected to be able to copy in the rainfall data and the <2.5mm days would automatically be filtered out of the calculation. the calculator even included some days which were recorded "0mm" of rain, this seriously skewed the result, we got a 90th percentile that was less than 1/2 of the what it was supposed to be. 

Has anyone else had this issue? My only solution is to make our own calculation file and sort from highest to lowest daily rainfall, because its not feasible to manually delete all the <2.5mm entries from >10 years worth of daily rainfall data. I am sure the LEED calculator has deceived many...