Count people and planned second meals
On the dinner row, list Pat, Lee, and one lunch for Pat. That is three intended servings. Now ask what kind of lunch is wanted: a full reheatable meal or a small addition to something already packed. This distinction affects how much you buy more than the word “leftovers” does.
Convert the count into a recipe decision
Use your household's experience with that recipe. A recipe's serving size is a starting estimate, and a main course may go further with bread or a side dish. After dinner, record whether the quantity worked so the next shopper can choose amounts with less guesswork.
If the recipe makes four servings, you can make the written amount and name the fourth use, or adjust it where the method permits. Decide which before shopping. “One pack for the recipe” is not a useful instruction when the shop offers packages that vary widely in size.
Learn from the next serving
Notice whether the planned lunch really existed after dinner. If everyone wanted seconds and there was no lunch, note that result beside the recipe. If a meal comfortably served four because of the side dish, record the side too. That gives the next cook a practical estimate grounded in a real meal.