It really depends on the usage in the model. If the bottle is viewable from 360 degrees, you wont be able to shade it very realistically since the light from such an item is dependent on the location of the observer. In such a case, it might be best to just make the bottle opaque if you're going for realism.
If you're set on making it half-full (which I think is a great idea), I'd try to narrow the angle from which the bottle is viewable. If the model cradles the bottle against himself, narrowing the angle to 60 degrees or so, you should be able to realistically paint a "translucent" item, since it wont be viewable from the sides from which it would look odd.