IMO, the location of the return line isn't nearly as imporatant as having good flow in the display portion of the tank. If the flow is appropriate, then the water stays churned just fine and every drop will get it's turn in the sump. I would highly recommend you just stick that return wherever it's going to "look the best" to you, since you're the one who's going to be looking at the tank the most. If you like the look of it in the middle, it'll work out great, ifyou prefer one overlow on the left and a return on the right side, than that would work great as well. I have one tank with a overflow in the middle and a return line on either side of the overflow, got one with overflow to one end of the tank, and return on the opposite, than I've got another I'm working on with overflows on the sides and a middle return likeyou are mentioning. All are just fine to use and all will work about the same in my opinion. Just make sure the display does not contain large dead spots with no current.