Howdy again,
Okay, after looking at your diagram, I have a few things to point out.
First off, you'll want your sump to be bigger than your refugium (probably a 20 gallon tank), because your sump will have to catch all the water that drains down from the tank when the power shuts off. Your tank will run about an inch or so higher when water is flowing through it, which must have somewhere to go when water isn't flowing. You'll also need to be able to catch the extra water flowing through the refugium, since it will also run higher when flowing.
Also, a Mag12 is overkill on a 55 gallon tank... you really don't want to run that much water through such a small sump or you're going to have a heckuva time controlling microbubbles, and the water will also be moving too fast for detritus to settle in the sump (which is where you want it to settle so it's easier to clean out).
You should shoot for 3-4x tank volume through the sump. Any other flow should be kept inside the tank to figure your total tank turnover. For instance, in my case I have a Mag18 as a return on my tank, though it's pushing against quite a lot of head pressure so it's probably only flowing about 900 to 1,000gph. The rest of my in-tank flow is made up with six MJ-1200 powerheads. I'm a bit at the high side for flow through the sump, but only at about 7.5x... you'll be at almost 22x flow. :bugout:
Also, the bulkhead in your refugium (I think that's the refugium) is way too low... you'll only have water in the fuge up to the first set of baffles (in the center)... nowhere near the whole ten gallons you're planning on.
If you give me a bit, I'll throw a diagram together to show you how
I'd plumb the tank... but of course you're free to modify it however you wish... I just think you're missing some of the principles of water flow when drawing your diagrams, so I thought I'd help you out.