from How to Cook Italian, by Giuliano Hazan
When a recipe serves 4 people but you want to cook for 2 or maybe 8 or 12, you don’t simply divide or multiply the quantities of ingredients. In general, the ingredients that make up the bulk of the dish, meats, vegetables, tomatoes, can all be multiplied, as well as herbs and spices, but ingredients such as oil, butter and wine should not be doubled or halved. The amount of these ingredients is determined not just by the quantity of the main ingredients but also by the size and shape of the pan. A good rule of thumb is that for each time you increase the recipe you should increase oil, butter or wine by only 50%, and if you are halving the recipe you should decrease them by only 25%.