I had a deja vu reading this thread and I remember you having asked basically the same question back then already.
Here's a C&P from my last reply there that might help with more complex shapes because it uses the already available geometry and does not involve shifting faces:
"There is another way, though, but it is more complicated than just "normal move" the liquid's surface into the glass:
- Create the glass vessel, create the cap of the liquid, then select the polys where glass and liquid would meet (involves cutting and connecting), and create a selection tag of them (after you joint them with the "cap").
- Assign a glass material to the whole object and a liquid material to the selection."
Hope that helps, if the method is still unclear, I can post an example file.
CU
Michael