I have been shopping around and have found a local store that carries Diatomaceous Earth and he says that the bag says "Food, Chemical, Codex Grade." Is this food grade Diatomaceous earth? Is this what I want to buy or not? I want to make sure it is the right stuff before I spend $41 on a 50 LB bag. Also if this is the right stuff how much should I use in a 8'X16' run and a 4'X8' coop?