A cooking ratio is a time-tested proportion that will produce the best results for many of the most common recipes. For example, pie dough is best made using a 3:2:1 ratio. That means 3 parts flour, 2 parts fat and 1 part water. A vinaigrette (1:3) uses 1 part acid for every 3 parts oil. You can easily scale up (or down) a recipe using the corresponding ratio.