Storm Pipe sizing

Hello everyone, so I've been plumbing for 12 years and getting ready to take my master's test in Colorado. I have questions when it comes to sizing storm drains. The 2006 IPC book had a chart that made it very easy to size storm drains which I will attach. It had rainfall rate, sq. footage, and slope you need for what size pipe you need to use. Fast forward to the 2015 IPC. All it gives you is GPM and slope of drain to determine pipe size. Is there something that I am completely missing. Any help would be greatly appreciated.
Thanks,
AJ Burns

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