I would place the top edge of the OF box even with the bottom edge of the top trim. So if you decide to turn off the pump for feeding or if the power ever goes out, that part of you tank wont cause the water level to drop below the trim. Exposing more of you tank then is needed.
I guess I should ask, how big a tank you have and how many linear inches you OF box is? How many GPH do you plan to run through the OF box? I can tell you roughly how tall the sheet of water will be.