Another Sump / Flow question

[ensquire]

Just finished up replumbing my 90 gallon tank. Turned the Durso into a herbie.
I have an 1 1/2" drain with a gate valve and a 1" return. Pump is a Waveline DC5000 putting out 1320 GPH at the pump. estimated 850 GPH at 4' of head height with a couple of elbows. I am running the pump wide open and the gate valve on the drain is almost closed to keep the water in the over flow at the height I want. I have tried this with a Quiet One 4000 with the same results.
I know the suggested flow rate is 5-7 times the total water volume but wouldn't it be better to have more than that if possible.
How much flow would one get through a wide open 1 1/2" drain ??