Make (or use) a small box that will fit a float valve, run your waterline to the float valve which is attached within the box. Affix the box to the inside of your sump at the level you want the water to be. drill a 1/4" drain hole through the box which runs right out through the sump wall. This hole should be the absolute highest point you want the sump waterline to get and should be above the float valve's shut off height.
Let's stop here so I can give you a visual of what we have so far. If you took a garden hose and filled your sump it would fill the sump, overflow into the box and then run out of the emergency drain hole in the box. Get it? ...good.
Now, drill a tiny hole like 1/16 or so low in the box below the level you have the shut off set to. When the water is low, the float valve drops, the box fills with water, and water drains out the tiny hole and fills the sump.
When the sump water level reaches the height the shut off valve is set at, the water stops draining out of the box, fills the box, the shut-off floats and shuts-off.
It is the differential in water height that causes the water to drain out of the box and into the sump. But... the hole is so small that if the sump water height reaches the height the shut-off valve is set at, the water in the box will not flow into the sump because there is no differential in water height.
Say the valve fails... the water from the box drains into the sump as normal, again the sump fills to the height of the box. The box fills even more to the point where it begins to run out of the emergency drain hole in the box. The sump will fill to this level but no water will exchange between the box and the sump. Because there is no differential in height. If fresh water continues to pour into the box it will simply drain out of the emergency hole.
The reef is happy and you sleep well at night.
