Hi Vladimir ,
no need for a custom Material on this , you can use the intersect tool for finding internal volume.
simply create a plane at the desired fill height on the bottle, run the intersect command, select both the plane and the bottle and create the internal body.
then using your evaluation tool check to see the volume.
EDIT : added video
intersect.mp4 1.9 MB
If you only need volume, you don't need to worry about material properties.
In addition to Scott's reply, edit the material on the part and look at how other materials are setup.
Depending how much detail you are looking for, density may be only what you need.