Overflow locations

[kkoch]

Setting up an acrylic 40g reef with overflows on both back corners, with X-mas tree strainers. How far below water level should I put them?

 

[heyfredyourhat]

I have my 2 drains on back wall, the top of the holes are about 2" below the very top of the tank. But i have enough room in sump to allow that much water to flow back when power fails