If you are using your own ground coffee - Use the normal basket (not the Supermarket Basket) we recommend between 22 and 24 grams of coffee; it will depend on the roast profile you use and how you grind your coffee. Double tamping is the best way to get consistent double shots.
Measure your coffee out, fill the basket and tamp. Then top up the basket with the remaining coffee and tamp again. The pressure for tamping should be quite forceful.
As with professional coffee machines, there is a little bit of experimentation to get what each individual likes as the perfect shot.
Note: If the coffee is a little older, then slightly more coffee is needed.
Also, grind size affects extraction and, therefore, the overall flavour and quality of each cup of coffee. When experimenting with grind size, make sure you only make very small changes to the grinder adjustments.
Light roasts are less porous than darker ones, so their compounds extract more slowly. So you will need to adjust the grind level.
When coffee is ground finer, it has more surface exposure. This means that extraction will happen more quickly. Remember very small changes until you get it perfect.
So if you are using light roast, grind it a little finer. If you using darker beans, use a coarser grind size.
That is precisely how professional baristas do it in a cafe.