The flow rate is controlled by the return pump. Water hits the overflow whenever it is too high. As long as the pump can't pump the water in faster than the overflow can drain, its the pump that controls it.
I initially bought a pump that was too powerful. Not too powerful for my overflow to handle, but too powerful of a return stream for my 55 gallon.
You don't need the CPR. You need some type of overflow that isn't a simple siphon, because you want that flow down to the refugium to stop if the pump stops. I've seen some interesting do it yourself overflows with PVC. But I'm not handy and luckily I'm at a point in life I can throw money at stuff.
I'd encourage you to at least think about a 20 gallon long tank instead of a 10, if you have the space. If I had it to do mine over, I'd steal space from the return area to make the central chamber larger.
Oh, and one other thing that makes a refugium great that I didn't mention: all evaporation happens in the return. All my water stains in the main tank are hidden behind my rim, since the main tank level never changes. And, when I top off betweeen water changes, I just pour it in the pump chamber. I don't disturb the tank residents.
EDIT: a decent write up on a DIY
http://www.bcaquaria.com/forum/tank-jou ... ign-24715/If you are going to build your own, the place to ask is at the saltwater huge tank forums. They've been doing sumps for decades. They seem to be pretty rare in freshwater situations, though I think they make a lot of sense for a large turtle tank.