Work Food Out lets you do basic math in the calories field to help keep track of how many portions you're eating. You can do addition, subtraction, multiplication, decimals, etc.
This is a very helpful feature for folks who weigh their food when they can. For example. If you eat some cereal that is 150 calories for a suggested serving size of 57 grams, and you eat 44 grams, you can divide the amount of calories per serving by the suggested serving size and multiply that by the number of grams you ate (150/57*44) and it will show you that you ate 116 calories!
Give it a try. And don't forget to work your food out.