Seems about right. I find on my Giottos double filter that if I am grinding very fine and using about 15 grams of coffee to extract 30 ml extraction over a 30 second period of when the lever is up, and I then use the shots for a latte or cuppachino has a bitter after taste. And the top of the puck seems a bit watery.
So now I am trying a coarser grind, tamping it down a lot harder, around 21-22 grams, and extracting less than 30 ml over the same 30 second lever up time, and the coffee tastes a bit better. Still not quite right though. But so far it seems to me that the Giotto requires a bit of overdosing, if thats the right terminology.
I also prefer the "look" of how the extraction is coming out of the machine at 21-22 grams coarser grind than 15 grams fine grind. The former looks like an unsteady treacle or syrupy flow whereas the latter looks like very thin but steady rats tails, if you know what I mean. ;D