Eshopps PF-1200 Overflow box

[Rocket421]

I'm using a Eshopps overflow box rated at 1600 gph running back to the wet/dry though 1.5" PVC and a Mag-drive 12 for the return pump. In order to maintain a decent water level in the tank the overflow has to be nearly fully submersed and the return pump restricted to about half flow. Is this a matter of the overflow box being underrated? Where does the waterlevel usually sit in relation to the notches on an overflow box? Thanks.