You can either pick the foods that have similar nutrient profiles or you can check body condition. I like to feed a wide variety of foods and I've learned with trial and error how much each of my six (I had eight but lost two earlier this year) should get. I weigh each meal with a scale (I have toy breeds so I use a tea scale as it is more precise for the smaller amounts). When I'm feeding a really low fat diet (like OC goat) I might feed a little more or add some extra fats to the meal - coconut oil etc. When I'm feeding higher fat raw foods I may give them a tiny bit less.
Feel your pups ribs and hips etc before starting the diet and if you notice a little weight coming on then decrease the amount feeding (or look for foods lower in fat). If getting a little ribby then add a little more. At least that's how I do it.