Amount per serving

If an item of food is stored for a long time, the nutritional values might reduce. Other processes such as freezing, thawing and cooking will also have an effect on the nutrition. Thus the figures given on the box are correct at the time of packaging. The figures at that time might be different from when you actually get to eat the food. In the case of your macaroni, it could absorb or loose water while in storage so the % values would only be correct at the time of packaging. Thus "as packaged" is a reference to the time of packing not the packaging itself.

As to the low fat content, dried powdered foods usually are low fat as it is difficult to make the powder with fat. They will probably have used a hard, low fat cheese such as Parmesan, or a cheese flavouring, rather than Cheddar.

Lose It!, a free weight-loss site, has this feature. To use it I think you need to start an account. To access it, you hit the Settings tab and then select Recipes in the left-hand column. Then select "New" and follow the prompts. This calculator allows you to enter the ingredients of a dish you are preparing. Most common ingredients will pop-up in the site's list with their respective caloric values. You can also manually enter in the values if you know them or if the ingredient is not in their list. You then set the "serving size" of your dish as its total volume. Then, when you enter in the amount you are eating into your daily food log, the site automatically shows you the caloric value of your portion.

The fifth screenshot here, shows you what it looks like.