DIY Sump Question

[LeanneP]

I am in the process of using a 20 gallon for my sump and my tank is not drilled so I will be using a siphon overflow. I actually have two questions. I am putting in two dividers and the tank is 16 inches high. How high should the dividers be as I know I need to leave room for water if the power goes out. Would 8 inches be a safe bet? I am also having my return pump go through my chiller but I am just wondering if my sump will fill up more every time the chiller turns on if it slows the flow of water down when it goes through it. Does this happen? By the way, I have a CPR Aquatic that does 600 gph for a 90 gallon tank.

Leanne