I let my water set and become the same temp as the environment my plants are in and then mix my dry
nutrients and have had no problem. I use both dry and liquid. in small growing situations and if you have to freight the nutrients in dry is better. In a metered dispensing system you will buy them in liquid so it can be dispensed correctly.
nutrient cost per gallon is cheaper to buy dry. It costs less and ships cheaper as you add the water. Liquid nutrients are premixed and require more space to ship so shipping costs are increased.
I believe you are referring to the liters of solution you are going to use in your reservoir. Lettuce use different nutrients then tomatoes and will use different amounts of liquid a day. Just as tomatoes will will require different lighting requirements then lettuce will.
Like my tomatoes and peppers use about 4 gallons a day at present. My new system will increase spacing and need more volume of liquid to fill. so as my plants change and mature they will consume more.