You have a "design liquid level" (DLL) - basically the bottom of your overflow pipe / slot in the tank.
If you have a floating roof, then this is limited by the roof depth to avoid any part of the floating roof going beyond the top of the tank wall.
Then there is a volume or level distance from DLL to normal max fill level (NFL).
This is decided by each operator / designer in turn.
Often the requirement is that at maximum fill rate there is at least 15 minutes from when liquid goes past the NFL before it exceeds the DLL. This allows sufficient time for the operator to do something BEFORE the tank over flows.
For very large tanks this can sometimes also be expressed as a distance which allows the tank level gages to recognize the difference from one level to the next (e.g. 150mm between normal, high level and high high level).
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
