When I dye I always use dye stock solutions. I never dye enough yarn to work just with the dye powder. And I want as little exposure to dye powder as possible.
Because I dye relatively small amounts of yarn I make 1% solutions. I use these solutions to mix the colors I am going to use for a project. The mixed colors are also 1% solutions. But it is very easy to make 4% solutions from these mixes. It is 4% mixed solutions that I use most often.
When I am dyeing small samples, however, I dye using 0.1% solutions. It is much easier to get reproducible colors when I do this.
I am not going to explain all this because Karren Brito has just done an absolutely marvelous job on her blog, Entwinements. On this post, she gives you all you need to know about concentration levels in dye stock solutions. She talks about what circumstances would lead you to use anywhere from 0.01% concentrations to 2.0% concentrations. And she explains in language that even I can understand the concept of margins of error. She also explains when it is better just to use the dye powder.
The book I rely on for my dyeing is Ann Milner's The Ashford Book of Dyeing. Milner deals with all different types of dyes and many different types of dyeing. It is Milner's book that taught me how to use the metric system. Karren's post fleshes out what Milner has to say about stock solutions and is definitely a keeper.
Karren herself has written an excellent book on shibori called Shibori: Creating Color and Texture on Silk. The book is not just about shibori. It is also about the acid dyeing of silk. I have found much there of value for my own dyeing technique.