This eye liner base recipe may be found in the book we sell, The Complete Idiot's Guide to Making Natural Cosmetics by Sally Trew.
In Sally's book she recommends that the color be about 10% by weight. For example, for every 5 grams of base (which would fill about 3-4 Slimlines, you would add about a half a gram of pigments. In our experiments, we tended to add more than that. More like 20% - 30%.