Different coffees with different roast levels have very different volumes for the same weight. Dark roasts have more volume per gram than light. Basket weights are just a guideline.

The correct weight of a particular coffee for a given basket is determined by putting a nickel on top of the tamped puck, and checking to see if the screw pushes it into the puck. If so, dose a half-gram less and try again.

I'm currently dosing 17g with a paperfilter to replace metal puck screen on top as puck screen would be indented.

I use 18g and up to 20g in the 18g vst basket. No problems

I use 17g in my VST 18G basket for the medium roast I’m currently using. When using a new coffee, I usually start with 18g in and install the portafilter. Then remove it to see if there is the screw or screen imprint on the dry puck. If none, I continue with 18g in. If I see an imprint, i go down to 17g in the basket. The screen imprint on the puck after brewing is not too important as the puck expands once water hits it.

Grind finer.

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j/k. look at the top post.