The following answer is applicable to all models in the Berkey line, provided that the volume calculations are adjusted.
The Big Berkey's advertised volume of 8.5 liters is the volume of the lower tank. This means that you can stock up to 8.5 liters of filtered water.
Explanation: The top tank is slightly smaller for safety reasons to avoid overflow if the bottom tank was not perfectly empty.
Specifically, in the case of the Big Berkey, the upper tank has a volume of 8.32 liters. In addition, the volume of the Black Berkey filters must be taken into account. Each filter has a volume of 0.51 liter.
If your Big Berkey is equipped with 2 filters, you can fill a maximum of 7.3 liters of water.
In practice, you will fill a little less than that because there is always about 2 cm of water left at the bottom of the tank (see explanations here).
You should also know that when the bottom tank has been emptied, there is also water left at the bottom, about 1 liter (see previous link).
The volume of water that you fill at the top has therefore been perfectly thought out and calculated by the manufacturer and allows, in current use, to have a reserve of 8.5 liters of filtered water in the lower tank, while avoiding the risk of overflowing.
Tip: if you want to empty the lower tank completely into a carafe, you can tilt your Berkey forward by grasping the lid button.