The German Coffee Association recommends taking 60g of coffee per liter of water. This also applies to preparation using the X-Tract method. We use a normal spoon as a rough measure; ours at home holds around 6g per heaped spoonful. This means as a guideline:
8 spoons for a full Arca (add approx. 800ml water)
5 spoons with 1/2 l water
3 spoons in 300ml water (a large cup)
If your coffee is too weak and it's not the amount of coffee, you probably brewed the coffee too slowly . It often happens with designer kettles with a thin gooseneck. The water flows too slowly and the coffee is not mixed. Then you have to stir the coffee well.
If the coffee is too strong, simply use a little less coffee powder or a different coffee roast. Try slightly milder and fruity roasts, perhaps with a coarser grind.