Here is a neat trick for calculating a bottles volume.
Draw a block over the bottle at the height of fill you want to achieve
When you extrude do not merge the bodies in your extrude feature
Go to insert /feature/combine
Then click on subtract and you will get the internal volume!
See the bottle i have attached
Yes it does as soon as you use the extrude as a main body and the bottle as the bodies to combineit will ask you what bodies you would like to keep and select the inside volume as the body to keep from there you can give the body a material like water and you have all the information you need!
You could add a sensor that could read the Volume for you whenever you switch to that config.
I haven't tried this yet but does the combine feature take into account the thickness of the bottle?