Modeling Exfiltration

I’ve designed a rain garden that includes both above-ground storage and subsurface storage via a stone bed embedded in a soil media layer. The rain garden itself is not designed as an infiltration system—instead, it is lined in impervious liner with a perforated underdrain within the stone media that routes flow to the primary culvert outlet. The rain garden is also equipped with an overflow device. Given this setup, when adding exfiltration as an outlet device over a surface area to govern the amount of flow to the underdrain, am i required to allow exfiltration for both of the defined storages? Also, how does this change if remove the underdrain and convert the rain garden to infiltrate?

Thanks