Overflow and Flow questions

[bearsfish]

I am likely not using the terms correctly so please (PLEASE) tell me if I am sounding like a dip...

Tank info - 40 gallon Corner Bow Front - see my build thread for more visual information

I am making an AIO section to my tank. I would like to know how to determine the width and depth of the cuts for the overflow into the first chamber. This chamber will house a rack that may have carbon in it and will have floss on the top to catch all the goodies (no skimmer on this tank).

I am planning on a 3/4" cut at the bottom of the divider from chamber 1 to chamber 2 and a 3/4" cut on the top of the divider going from chamber 2 to chamber 3 (the return).

At this time I am planning on using a smallish (450 l/h) powerhead to run this part of the system. Just enough flow to keep the tank warm (heater in chamber 2 or 3) and keep the water surface clean.

Do I need a larger pump? I have a 5 inch square chamber so size matters.

My powerheads are plenty large enough to create the flow in the display chamber so I am not really worried there.

 

[bearsfish]

I have an additional question considering that the 3rd chamber will ultimately control the actual level of water in the rest of the chambers - should I have the overflow cut lower to ensure that the water will overflow properly?