When calculating recipe and portion costs, allowances need to be made for unavoidable waste when calculating the recipe yield. Failure to do so will result in understating portion costs. Subsequently, instead of figuring on 100% yield from a recipe, assume that only 98% will be actually used, allowing 2% for evaporation, over portioning and quality control waste. This percentage of waste will differ depending on the menu item. There is a higher allowance for waste on items prepared from scratch than on items that are purchased pre portioned.