Originally posted by murambi
dont quite understand your explanation
What I was saying is that you can insert an edge loop on the inside of the glass (depending on how high you want the glass filled) and extract those inner faces to make the liquid, extrude inwards those border edges to close it up and merge the verts. It avoids the "air gap" effect and makes it look more accurate.
Check it out
Autodesk Water and liquids - help files
It says three surfaces but I cheaped out and left it at two :x