Take the surface area of your tank (length x width) and divide it by the surface area of your sump (length x width) and that gives you the inches required empty in your sump per 1 inch that your display drains when power is shut off.
With that said, I do it the way @Reefering1 does. I fill the sump through the display once the plumbing is in place and stop filling when the sump is at my "comfort level". Also, leave some extra room for ATO malfunctions as a just-in-case.