Custom property of assembly feature

Hi all,

I am designing a set of pressure vessels and I want to display the exact internal volume of the vessel on the assembly drawing. 

 

Here's where I'm at:

  1. Created a part that is larger than the largest vessel in the range.
  2. Used an assembly feature (revolve cut), linked to the vessel geometry, to shape the part to the inside of the vessel.

When I measure this part in the assembly it's calling up the correct volume. How can I create a custom property of this so I can display it on the drawing. All attempts so far are just returning the full volume of the initial part created in step 1.

 

Alternatively, is there a better way?

Any assistance would be appreciated, thanks. 

- Steve