Volumes of Tea-Cups

The standardised volume of a drinks cup is 300 ml. A quick measurement of the cup I drink tea out of at home confirms that 275 ml is enough to fill the cup to a normal height (that is to say, with about a centimetre of air in the top). Faith's comfort cup (a shorter cup with a wider bowl) takes the same 275 ml. So measuring out 550 ml into a measuring jug and boiling that up makes exactly two cups of tea. Loss of water through boiling appears to be negligible, so this gets us a round of tea as quickly and cheaply as possible. Your cups may vary, but 275 ml of water per cup seems to be a pretty good rule of thumb.