For feeding the brine shrimp, I use spirulina, some spell spirolina, which has been crushed up in the bag, and placed in the centre of a piece of dense cloth. I fold the cloth up to form a spirulina ball and secure it with an elastic. I soak the ball in saltwater in a mug and refrigerate it overnight. Over fog just once and the whole container is wiped out. If you underfeed, you only loose part of the culture to starvation.
Experience is the only way to find out how much to feed each time.
I'm now using Brine Shrimp Direct Tahitian Blend and find it more convenient, and better still, the yields are higher. I remove some adult shrimp with a regular fishnet, rinse in freshwater, and feed to the fish that prefer adult sized shrimp. Using a regular net leaves the new live born shrimp for future growth. Adult shrimp produce live born when conditions are right, and cysts when water conditions deteriorate. If you find an easier way to grow adult brine shrimp, I would appreciate hearing of it.