the website has a calculator. for step 1, put the # of gallons or liters you want to mix up, and step 2 you click whether you are measuring in gallons or liters for the # you entered in to the calculator in step 1. leave step 3 alone.
so for 3 liters, i would put:
the number (#) 3 in the input field in step 1
and i would click liters for step 2
and i would leave step 3 as it is.
like this
and it will tell you how many grams to add to your chosen nutrient amount entered (in my example it would have been 3 liters of nutrient solution). hope this helps you out.
edit: don't measure in cups as the website shows, as the measurement would be hard to be precise measuring using a cup.