Ok, that changes things a little but, not in a bad way. Your overflow from the main tank will be set to a certain level, let's say for discussion 3" below the lip. Now things are flowing into the refugium and the pump returns it to the main tank. If the power goes out water will only drain to the level of the overflow. Once it goes below that it can't drain any more. Now the thing to do is to size your refugium to be able to hold all the water that could possibly flow into the refugium if water drains to it's lowest level in the main tank. The size will be determine by the depth of your overflow and size of your tank.
For example, you determine that if the pump stops your tank will drain 5 gallons. Fill your refugium that will hold 20 gallons to the 12-14 gallon point. If all 5 gallons drain, they your refugium will still have some room before it overflows.
This is essentially how all sumps work. Just need to have enough volume in your refugium for the worst case scenario.
Make sense?