Go to a couple of the 'curb inlet' manufacturers' websites and read and read and then watch water in the gutter. Then go to your largest local Public Works design group and ask how they do it. There are theories but they depend on perfectly placed and finished gutters. That never happens.

FHWA's HEC-12, available for free download at

http://www.fhwa.dot.gov/bridge/hydpub.htm

summarizes a method I have never seen rejected by any reviewing engineer.  Pages 22-35 details all equations you need to create a spreadsheet to do this.  Composite gutter cross sections are included, which I think is what you are looking at.

There is also a free software download available, but it had no tech manual with it when I looked at it (years ago, that may have changed); so I did not know where the numbers where coming from, so I didn't feel comfortable using it blindly.

Visual Urban is a Freeware program that automates the HEC-12/22 from FHWA.  It can be downloaded from the FHWA Website.  I have compared results with the nomograghs and equations from hec-12 and it produces acceptable results. As stated above sreadsheet can be coded with the equations and easily viewed for your study.

Good Luck & Have Fun

Neenah Foundry Co. has done actual tests on their frames and grates.  Try their web site or give them a call.

Maury makes a good point.  A high-efficiency grate is probably going to help more than gutter geometry, and Neenah tends to be very responsive.  Make sure it meets bicycle-safe requirements...maybe fax a cutsheet to the reviewer and/or your DOT.