Ah! So now I know. But since the actual amount depends upon the diameter of the glass (and the diameter of the finger), it's not terribly accurate or easily reproducible.
I don't measure much when I cook either unless it's a canning recipe or baking, especially bread. My great grandmother could create cakes that were so light they'd almost float away and she never measured anything. I remember her version of measuring something would be to use a clean tea cup to dip into a flour sack and throw that into a bowl. If I tried that method I wouldn't have a cake, I'd have a door stop.