Hi B., 

Thank you for your question. My opinion is that it will be difficult to make a nutritionally complete feed for your chickens at home. Chicken feed contains 5 basic components: carbohydrates, protein, fat, vitamins, and minerals. While putting together mixes of grains and legumes is easy, it would not be a complete feed without the addition of the proper amount of age-specific (chick vs young hen vs older layer) vitamins/minerals. Additionally, if the grains are not mixed in the proper proportions, the feed can contain too much or too little protein, carbohydrates, fiber, or fat. The risk you run when creating your own feed is not providing the nutrients in optimal amounts to allow for long-lived, productive birds. For instance: too little protein can result in small or no eggs and feather picking issues. Too much protein can result in money wasted and increased nitrogen excretion, which isn't ideal for the environment. Improper proportions of calcium, phosphorous, Vitamin E, and Vitamin D have the potential to lead to weak shells and bones. Too much fat or carbohydrates can make hens fat, which can result in prolapses. 

Standard chicken feed is formulated by poultry nutritionists, takes the guess work out, and ensures that the feed provided is nutritionally complete. If store-bought feed is stored in a cool, dry place, it can last for about a year from the time of purchase. I have also seen smaller bags of chicken feed (5-20 lbs) coming on the market for very small flocks like yours.

More information on poultry nutrition and feeding can be found in this video by Dr. Jacquie Jacob. While it doesn't go into mixing feed, it does explain some common feed ingredients as well as their pros and cons.