While I would switch over to using SCS instead of rational, there is a crude way to get close.
Rv = 0.05 + 0.9 * IA
Where: Rv = Runoff coefficient (unitless)
IA = Impervious fraction (unitless)
DV = 3630 * RD * RV * A
DV = Design volume (cu ft)
RD = Design storm depth (in)
A = Drainage area...
