I'm working on a green roof over a building in IP and want to understand what system (extensive, etc)would be better.

I was able to calculate the 95th percentile of my site (46mm) and the Small Storm Method (14,80, considering a 30% impervious area).
Then I've developed a complicated spreadsheet showing the green roof depth, substract weight dry and wet, drainage type, etc, in order to understand how much runoff the green roof can handle, but I think I have a big chance to lose some point in here. My question is: it's there a standartized procedure, since I haven't found anything on the reference guide? Thanks!