Chemical measurement questions
Generally a specification like 3% of potassium sulfate is wt%. Such a solution would have 30 g in a about a liter of water (weighing 1 kilogram). $$30/1030 = 2.91 \%$$ which should be close enough. You could use 970 g of water to be more exact. But just filling a beaker to the 1 liter mark would work fine. 

I never though of cooking in Europe, but I'd guess that they do have measuring spoons in some $\mathrm{cm}^3$ (mL) units. Again good enough would almost certainly work. $$ 1 \text{ teaspoon} = 4.92892 \text{ mL}$$