Mysha - As a one time scientific technician, I am perfectly capable of devising means to accurately measure volumes of containers. that wasn't what I was saying.

My point was that the shape of most common pub glasses is quite deceptive as regards the volume of beer in the top "slice" (whether deliberately commissioned by the pub chains for that property, or just an artefact of the design that fits best in glass washing machines).

In general we are used to judging volumes in parallel sided containers where half the volume equates to half the height. Because of this, we expect the "top slice" to contain less of the overall volume than it actually does. For judging 5% of a glass of beer, before I was aware of the actual amounts, I would have looked at the glass - my brain would have mentally divided what it saw into ten equal depths, then divided the top one in half again. The resulting mental estimate is a much thicker "slice" than the one which actually contains that top 5% of the volume, hence I wouldn't realise how much beer I was getting less than the amount I ought to have in the glass.
Quack!
GtD.