With roux, the longer it is cooked the thinner the consistency gets. White and blond roux are much thicker than a brown or dark brown roux. A standard roux calls for a 1:1 ratio of flour to fat. When I make a dark brown roux for my gumbo, I will up the flour called for by about 25% to a little extra body to the gumbo.   For example, if I would normally use 1 cup of flour to 1 cup of oil in my gumbo roux, I will use 1 ¼ cups of flour to 1 cup of oil.