Why is it that no matter how large the inlet on grade is the gutter spread always remains the same?
HEC-22 inlet computations are concerned with when the gutter spread is maximum, which is immediately upstream of the inlet (see image below). Just downstream of the inlet, the spread is at its minimum because most, if not all, of the water has been intercepted. The spread is calculated for the flow in the gutter immediately upstream of the inlet, so the size of the inlet does not impact the width of the gutter spread. To remedy this situation, try increasing the size of the inlet(s) upstream of the inlet you have been working on.
Order of automated design priorities for StormCAD (GVF-Rational) and SewerCAD (GVF-Convex)