Cut the shape of the tank with a piece of balsa or other solid wood. Then
place it in a tub of water until submerged and measure the volume of water
displaced. If you have something like a laundry sink, it is pretty easy
math.

Even easier is to build it into the shape that will fit into the VP and then
measure the liquid it holds and rewrite the manual.

As they proved with the Hubble telescope, you cannot really trust the math
guys, anyway.

Colin