I usually mix my nutes by the gallon, as well, 'cause I'm American like that, lol. But honestly, I like to start by making a larger batch of nutrient solution, say, 4 gallons worth at least. I find it's a lot easier to measure out a larger dose of the nutrient solution; just multiply the nutrient's per-gallon dose by the number of gallons of water. I use a liquid measurement conversion calculator online (just google it; "milliliters to teaspoons conversion calculator", for example). You select the unit of measurement you're converting from, the unit of measurement you're converting to, and the value you wish to convert. Sometimes it's just easier for me to measure out appx. 4 teaspoons of solution rather than 20 ml, you know? Then once it's been mixed, I just have to measure out how many gallons of it I need. I always try to mix the exact amount of nutrient solution I need fresh each time, but if I need to store some for later, I keep it in an opaque plastic container in a cool dark place, using a simple fish aquarium air pump attached to an airstone inside the container to keep it oxygenated and help prevent the mixed solution from becoming stagnant until I need it.
Hope that helps ya a bit! Happy growing!
