My calcs show:
Initial Flow = 834 cfs
Reqd Flow = 834 x 1.2 = 1000.88 cfs
A depth of 5.8 ft. gives a flow of 998.99 cfs, very close, but not quite the reqd 20% more than the original flow.
A depth of 6.1 ft. gives a flow of 1059.37 cfs, which is 27% more than the original flow.
I thought...