When I did my leak test, I filled the DT with tap water until it just started to drain to the sump. Next I filled the fuge until it started to drain into the return section. Then I filled the protein skimmer section until it started to drain into the return section. Finally I filled the return section to about 1/3 full and started the return pump in the lowest setting. Keep the return section at 1/3 full with water as the pump pushes the water to the display tank until the water starts to drain from the DT.
Once the water level is stablized in your DT and your return section is no higher than 1/3 full, mark the water level in the return section and turn off the pump and see how much water will drain into the sump. As I said if you glue the standpipes to your bulkhead, only 2-3 gal of water will drain to the sump. If you just dry fit the standpipes to the bulkhead, all the water in each cornerflo will eventually drain to the sump since it is not water tight. This may take many hours.
Mark the sump when you think all the water have drained from the DT. The difference between the first and second mark will be the volume of water that you need to account for in the event of power or pump failure.