Yes, but you are missing the final step. When the pump comes back on, the water level in the display tank will rise to replace what has drained from it, but that in turn will lower the water level in the sump.
For redundancy and avoidance of flooding you need to consider two worst case scenarios:
1) the weir blocks and the return pump pumps all available water in the sump to the display tank until the pump compartment runs dry
You want to ensure that the display tank has capacity to take that additional water volume. Hence
@zozo 's recommendation previously suggesting raising the pump up a bit in the sump, and running the water level above it to a volume that the display tank can take before the pump runs dry.
2) the display tank drains to is minimum physical point (this could be to the bottom of the weir, but if the weir fails, it could be to the bottom of any outlet holes cut in the tank walls)
You want to ensure that the sump has sufficient spare capacity to take this additional water. Now you are adding an overflow to the sump, essentially giving your sump infinite capacity, but also automatically draining away any excess water that flows from the display tank.
So lets say you want your usual water level in the display tank to be 25mm from the top rim. This is around 30 litres on a 180 x 70 tank.
Say for arguments sake you have a large 100 x 40 x 40 sump, and you don't have
any dividers, so the water level is uniform through the entire sump. (This gives the largest capacity in this scenario - if you have dividers/ compartments, then this scenario is even worse).
To eliminate the risk of flooding the display tank, you run the sump water level just above the pump as
@zozo explained in his post above. You know your display tank can only take 30 litres, so in your sump the water level needs to be no more than 7cm above your pump inlet (100 x 40 x 7 = 28 litres). So this is where you place your operating overflow for the continuous drip water change system.
Now you immediately have a bit of a problem.
Say when the pump goes off (say a power cut), you lose 2cm of water from your display tank (and this doesn't account for failure of the weir) - that's around 25 litres. That goes down to the sump, and out of the overflow. The pump then comes back on, and sends 25 litres back up to the display tank. The water level in your sump is now basically at the pump level, so it'll be drawing in air and dry run protection will kick in or the pump will burn out.
Now you could have the operating sump overflow higher than 7cm above the pump, but if your weir or drain pipes get blocked for any reason the display tank will flood.
