Start with a cup large enough for the amount of drink you'd like.
When the cycle finishes, and the plus sign comes on, press and hold the button for extra water. Note that the water doesn't start pouring immediately (the disc was empty, now refilling), so it won't stop immediately, either! Allow for this by releasing the button a least 2cm before the cup is full.
You'll need to stay by the machine for this option: the green plus light doesn't stay on forever.
11 months ago