Another way to place your powerheads is so that the waterflow creates a GYRE. This is done in the best manner so the the general flow of the water is circular, with the surface water heading into your overflow. It is best for nutrient export, you also want to randomize the flow, which replicates the ebb and flow of wave action in the ocean.
IMO placing powerheads about halfway down the back pane of glass one toward each end of the tank(or more) aimed in the same area of the front pane is a great way to achieve this. The lower you can place them the better in my opinion, as long as you can make them look good and not kick up the sand.