If you want to model an inlet the correct way, that is computing inlet capacity, Q capture, bypass and carryover as well as gutter spreads, you need to use the right tool for the job. Most reviewers are going to expect you to produce those results. Hydro-cad wasn't designed for that. You'll save...
I agree with RWF7437. This is total misuse of the Rational method. Despite its heavy use, it was never intended to be used in a Q vs. Time relationship with multiple sub-basins as it suffers from the "Partial Area" affect. To offset this character, some practitioners employ, as Andrew Reese...