This is usually dictated by local ordinance and practice. Design storm events and allowances for onsite storage and offsite impact are often unique to the area. If your area is covered by a regional water management district, the will also have a say as to design criteria and "downstream" affects.

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.