re eating packaged food, you have to watch the number of servings on
package and if it says there are 4 servings in the
package and you eat the whole package, you've eaten 4 servings and you have to multiply the calories/serving times 4. It's best to not eat package/processed foods, because they tend to have the most calories.
If you stick with lean meats, eggs, dairy and bean/legumes for protein and fresh/frozen veggies, you don't have to worry so much about counting calories.