Depends on what product you're using, and the number of products. They do have a nutrient calculator on their website. You figured out how to sign up here, I'm sure you're capable of using it too. Generally speaking I use about 40 ml of each Sensi Bloom A and B per 4 gallons of water, or about 10 ml per gallon. This rounds out to close to 2 ml per liter so it sounds like your friend is right on.