I would just go by the directions on the bottle---I think someone here said that it was one ounce per gallon of water. That would be the correct concentration, no matter the number of chickens. I think that has to be the only source of water for a period of 24 hours. After 24 hours have passed, you will want to replace the wazine water with regular water, and toss the eggs for two weeks. If it is just round worms, you may not need another wormer. I would watch the poo, and make sure that they are clear of the worms after using wazine.
There are organic means of keeping chickens free of worms, but I am not sure exactly how successful they are. I don't think you can treat with those means, only prevent. Occasionally you will find an article on such preventions in Poultry magazine.