Hi, whatever coffee i use, when i put 15 Gr of coffe on my VST 15Gr i’am always 1 or 2 mm under the line after tamp
Apparently this is the reason why i get thoses impacts after a shot (too much space when awter come)
Of course i can use more then 15 Gr, but is it the same for you ? i remember that Matt Perger, use to say put 15 Gr in a VST 15 Gr 
Thank’s
Hi Stephane, The ‘headspace’ between the fresh tamped coffee and the shower screen is the important thing here.
15g of high density coffees like: washed, high grown, light roasts will sit lower in the basket than 15g of low density coffee: low grown dark roasts, giving different head space.
Illy in their Espresso book say 5-6mm is a good head space, I find anything from 3 - 6mm is ok for light roasts.
You can check how much space you have by putting a thin coin on top of your ground coffee then inserting the portafilter and immediately bringing it out again to see if the coin has been pressed into the coffee.
Do not worry about the line, it is designed to hold the basket in not act as a guide for dosing. Different machines can have different heights that the shower screen hangs down, though once you know your machine it can be used as a useful marker.
It is difficult to learn anything from the used puck, it gets marks and impacts when it expands after the pressure is released (turned off)
hope this is useful
Gwilym
I think it’s about the headspace.Or maybe you can try a finer grind ?
Hi Stephane,
I 2nd @Gwilym’s comment below about the ridge in your basket and that it’s not meant to be any type of indicator for dosing or an optimal target. I like to stay within 0.5 g of the basket’s stamped size and use both 15 g. (dosed to a max. of 15.5 g.) and 18 g (again, dosed to a max of 18.5 g.) baskets regularly at home depending on what I’m preparing. Used pucks are not very good at diagnosing extraction problems, but I do see a couple of spots here where it looks like the water is penetrating the top of the puck which could lead to channeling (that’s neither here nor there with respect to this post…).
Regards,
S.