The 1mg/liter is the final concentration when diluted in water(after ethyl alcohol) and not the dosage.
There should have been a measuring scoop included. According to National Fish Pharmaceuticals, for pure Praziquantel powder, it is 1/4 teaspoon per 40 gallons. There is no other description of dosing pure powder besides that. With a teaspoon, you get roughly 4 grams per teaspoon(this is with table salt so weights would be different)I would stick to the 1/4 teaspoon per 40 gallons.