InRoads Storm & Sanitary will compute the exact same spread with these values entered above (see image below) for on-grade spread (equation 4-2). So I believe the difference you are seeing is because the program you show above is not distinguishing between spread calc for on-grade and spread calc for sump. If the inlet is in a sump, the longitudinal slope is 0.0%, and equation 4-2 can no longer be used. Then it becomes necessary to use the methodology from section 4.4.5 “Interception Capacity of Inlets in Sag Locations”.
If you wanted to model the inlet in S&S in order to match the calcs you show above from the other program, you would need to change it to On-Grade, make it’s transverse slope 8% and it’s longitudinal slope 0.2%.
