The amount of drain down will depend on quite a few things. How much of a water level change do you have between operating and no pump on the tank? IE with a higher flow return pump you will have more drain down. How much water does your return line hold? Is it 3/4, 1, 1 1/4, 1 1/2, 2 inch, how long? You should be pretty safe as triggerfreak says, but without knowing how much your tank drains. The sump will be able to hold an extra 26.5 gallons of water, so assuming you have say 3 gallons in your plumbing that leaves 23.5 on the main tank, which is right at 3 inchs in the main tank. But what about your skimmer, will it drain down? Do you have a seperate fuge?
The sump will normally have about 31 gallons in it I figured.
The overflow will be 15 gallons.
The 48x16x16 sump can hold a max of 53.2 gallons.
So 31 + 15 = 46 gallons when the power goes out, so I should be in the safe range.
Cant make any mistakes like that before I build the sump.