in a cafe, it's probably different, but at home, i measure by the appearance of the crema. I used to pull double shots with ahlf going into a measuring shot glass, and pull the shot til it reached the 30ml mark, including crema, before it settled. The crema was often fairly blonde and the coffee was often watery. Now i'm grinding finer and packing a bit more into the basket, which means that my shots taste richer, and are shorter ... i'm getting about 35ml double ristretto's (not measuring anymore), because i'm ending the shot when the pour starts to show blonding. My point is, to me, the liquid amount is a guide, not a rule. If it tastes better, it doesn't have to be 30mls. probably different in comps though - glad i'm not competing!