Melevsreef is awesome.
The overflow won't spill your sump because it will drain water out only down to the bottom of the box teeth, and no further. Without a pump returning water to the display tank, the overflow can only take a finite amount of water out before running dry: it's self-limiting by design. Another way to think about it is that the overflow can only take out as much water as the return pump can put back in, while the return pump only has as much water to pump as the overflow can put into the sump.
What you basically want for insurance is to have enough room (and some to spare) in your sump to hold the extra volume of water that your overflow will drain out when the return pump is turned off. When set up properly, you won't have to worry about your overflow flooding the sump when you turn off the return pump (or have a power outage). It will just equalize and self-limit naturally. You won't need to do any sort of expert timing to start it all going either. Turning off your return pump shouldn't be a hassle or a danger.
Your main flood worries will actually have to do either with the overflow not being able to keep up with the return pump, or the return line acting as a siphon:
1) if the overflow gets clogged, is too small to begin with, or its siphon breaks while the main pump is turned on, the return pump will first flood your tank with extra water in the sump, and then run dry, eventually damaging the pump. Clogs can be caused by long-term buildup of gunk in the siphon tube (it's a good idea to screen it from light so that algae won't grow inside the tube) or even critters that wander or jump into the wrong place. Siphon breaks can be caused by too many bubbles going through the siphon, getting stuck in the top U bend, and steadily reducing the volume of water the siphon can pull. Overflows are all rated as to the gph they can handle, so try to match this with the power of your return pump (taking into account that ever foot of height the pump has to cover reduces the gph output of the pump some)
2) If there isn't a proper siphon break (i.e. a hole just at or under the stable water level that can suck air and break the return line's siphon when the pump stops pushing) in the line that returns the water into the tank, then when the pump is turned off, the return line will flow backwards: sucking water out of your tank, and flooding the sump. Bad bad bad (especially if the return line's nozzle is deep in the tank: it will drain until it hits this level, which can mean many many gallons spilled) Make sure to test your siphon break system by turning off the return pump and watching what happens (i.e. once the overflow stops pulling water, the level in the sump should NOT keep rising further!). Sometimes the holes can be too small, and fail to break the siphon entirely.
If your skimmer makes microbubbles, you might want to consider having its output spill into a bubble tower instead of straight into the open sump. Melev has a bunch of designs for these on his site. I built one for my RDSB/bucket return out of a simple 2 inch thick PVC piece with teeth dremeled out on the bottom, and a hole through a cap on the top where the water flows in: costs almost nothing, but dramatically cuts down on bubbles.
If you are thinking of having cheato in future, you'll want as big a sump as you can fit, with a big section of it devoted to being your macroalgae fuge. You'll also want to be able to isolate light in this section as much as possible (so it won't spill into the DT or even other sections of the sump if you can help it: extraneous light = more extra unwanted algae to clean). Again, melev has some great examples of what you want in a fuge section (getting flow for the cheato is important, as is making sure the chaeto stays in the fuge section rather than floating under a baffle or away.
