Can't resist throwing my usual "simpler is better" oar into the water here.
You know the old adage that claims there are no dumb questions? That's utter nonsense; there are plenty of dumb questions, just look around this forum to prove it...but yours is not one of them.
Obviously, you can do what many others do, using valves and variable speed control pumps and assorted doodads to *balance" input/output and "dial in" your sump.
It can also be much much simpler, especially if you are willing to drill your tank. Use an online chart to determine the size of pipe that will comfortably handle your pump's flow rate. Put in at least two bulkheads that size, as close to the top of your tank as possible. You can always add elbows that extend down to the bottom to pick up water from the lower level, but you always need to have a breather hole drilled at the top to prevent the formation of a siphon.
The outside of the bulkhead is simply plumbed down to the sump, no valves or other gadgets needed. The pump can be run at full speed...in fact, all of mine are of non-variable-speed design. If I have a pump that puts out X gph, I guarantee I will find a use for every last one of those gph, no "dialing down" desired or required. If for some reason you think you have too much flow...you can always use T's/manifolds/valves to split the output and direct some of it elsewhere...a protein skimmer, a "reactor" of some sort or even a simple directed jet to create surface aeration or keep areas of the bottom clean of accumulated debris.
Some folks say that big powerful pumps, run at halfspeed or whatever, will last longer. Maybe that's true, I dunno, but I have had no longevity problems with short-lived pumps being run at red-faced maximum 24/7. I buy pumps that have only two speeds: On and Off. But, heavens, if I want to turn them On or Off, I actually need to unplug or plug them in; I can't phone them and tell them what I want them to do. Disgusting, huh?
Make sure the sump is large enough to contain all the "flowdown" from the tank when the power goes out. Make sure the pump chamber is large enough that it isn't immediately pumped out if the overflow drains in the tank are momentarily plugged, perhaps by you during some maintenance, perhaps by accident. With a redundant second drain, this is virtually impossible. Too small a pump chamber can create problems if you remove a large rock or decoration, or perhaps scoop out some water to remove some fish...a small chamber can have the water level dropped far enough for the pump to start sucking air.
Remember that the pump chamber is the only place in the whole system that changes in the water volume will actually be visible. A 6x2-foot tank has 12 square feet of water surface from which evaporation can occur...but if the pump chamber is 1x1-foot in size, the water level in that chamber will drop twelve times faster than you are expecting, while the tank will remain "full".
With an overhead sump set-up, the display tank is actually the "sump"; the pump, or at least the water intake, will be in the tank. When using one, you need to make sure there is enough spare volume in the display tank to contain all the water draindown from the overhead chamber when the power goes out. Depending upon the size of the two units, this might require you to keep the water level in the display tank lower than you might want it to be for cosmetic purposes.
Sumps make perfect sense when you use them for awhile. PITA to try to explain; easy to understand when you're looking at one. Lots of ways to do it...but do it! You will never go back to leaky squeaky sealed canisters.
