Use HEC12 Chart 11, choose which flow is present.
For weir flow, depth = [Qt/(Cw*P)]^(2/3), ft
P=perimeter of grate, I use 3.0 for Cw
For orifice flow, depth = [Qt/(Co*A*2g)]^(1/0.5), ft
A=effective flow area of grate, I use 0.67 for Co
lha is correct. If you go to the fhwa web site you can download visual urban which is a program that calculates this type of headwater and has a decent print out for the results.
