I did my computation in a backasswards way, but it worked.
We know that gallons = width x height x depth divided by 231, right?
So I figured how much water my display tank held per each inch of height- 48" long, 18" deep, and 1 inch of height (divided by 231) = 3.7g.
So I need 3.7 gallons of space in my sump per inch of water drained out of the display.
My sump per inch is 48" long x 12" deep x 1 inch of depth (divided by 231) = 2.49 gallons per inch of water.
So for every 1 inch of water that drains out of the display when the power goes off I need 1.48 inches of room in my sump.
Now- how much water will drain when you shut off the power?
That depends on the height of your overflow, where you put your siphon break, and maybe other things- I don't know. I figured 2 inches to be on the safe side, so I need room in my sump for an extra 7.4g when the power goes off.
Knowing that each inch in my sump represents 2.49g, and I need room for 7.4g, I divided 7.4 by 2.49 and knew I needed an extra 3 inches of room in my sump. I find that I have lots of room to spare.
Told you I did it in a convoluted way.
There might be a chart for this somewhere, but this is how I did it.
Edit: to clarify