Its a 46 gallom bow front, seaclear system II. So 40G display volume 6 gallon behind. I chose that position because I have an L 10 hole manifold on the right hand side behind the rock work. I tried two powerheads left and right, I tried one left, I tried one right. Every position possible. The best flow was a single power head middle. It removes detritus the fastest and leaves no top or bottom dead zones for my tank. Plus it encourages the fish to use both sides of the tank more than other positions I tried. Now they hang out equally on both sides.
I got the idea after looking at tanks with center overflows, they always do left and right wavemakers. So I figured I got a left side overflow, and a right side return, I need to split it in the middle to maintain flow. It worked. Took several hours and my fingers getting pruned to find the perfect position. I used finely ground flake food to test the patterns of the flow, and cleaned pre-filter when I was done so none of it stayed in my tank.
Note: power head is angled down not up. That is the only way to make it work properly for my tank. splits the flow when it hits the front acrylic left right and down. Forming two circles on either side, and because the return is right side it forces surface detritus left towards the overflow.