[mark]

backflow; it's going to be the volume of your overflow chambers (or less if you have a standpipe in them), volume of your display until you reach the siphon break of your return outlet plus the volume in your lines.

Best to fill your tank until it overflows into the sump, then fill you sump to your max height your comfortable with. Turn on you pump and mark the level that your sump settles down to as something like "Max fill, pump on" and adjust you ATO to the level as well. If you never top up above this mark with the display running, you won't flood.

As said every display will be different amounts because each each tank will be a little different. My case I'll backflow ~26g but is easliy handled since I run my 75g sump little over 1/2 full.