sizing of storm building drain

[Mac Moonfire]

I'm trying to size storm building drains from BC plumbing code 2018.
Table 2.4.10.9. and the notes are easy enough to calculate the potential hydraulic load based on the roofs area.
I have a site where two buildings are discharging both gutters and foundation drainage into a main that end in ditch.
I don't know how to calculate the hydraulic load of the drainage system. It seems very site-specific anyway
Thanks

 

[Paul Sweet]

Don't connect the foundation and roof drains. Blockage downstream of the connection will force water coming off the roof back into the foundation drain.