The teeth on my overflow are 2" below the rim of the tank so I figure when the pumps are off 2" of water will drain back into the sump using this calculator
Aquarium Calculators that equals out to about 12 gallons, and if i make the baffle in the 55g sump 6" from the top it will allow 15 gallons to drain when power is off. so i will be heading to the glass shop to get those baffles cut later.