I use the same amount of water in a 2 liter bottle with the bottom cut out & inverted in the cut off bottom. All you have to do is vary the amount of bbs eggs. I'm guessing my setup holds around 1 1/2 liters of water. I use a tablespoon of salt, around 1/8 teaspoon of baking soda, and anywhere between 1/8 to a full teaspoon of eggs.
I read somewhere that a fish's stomach is around the size of it's eye. I pretty much use this to figure out how much to feed. I don't think this would work for squid, having enormous eyes, or cave dwelling fish, which have no eyes at all. Seems to work with livebearers, angels, and assorted corys & plecs for me though.
So I guess 4 eyes worth of bbs should work for 2 fish, 2 feedings a day. Any time I have extra I feed it to the rest of the tanks.
HTH!
Tolak