As far as the parking lot drainage itself. I work in a very flat area and for asphalt parking I try to have 2% slope for sheet drainage, 1% is the absolute flattest I will go. Concrete parking I try to have a minimum slope of 0.50% for sheet flow and 0.25% for longitudinal gutter/channel flow. Once again, I am in an area where natural prevailing slope is less than 0.1%.
We don't have any detention or mitigation requirements for downstream impacts, but that is something you should consider if required by the jurisdiction.