For your first question:
There are 4 calories per gram of carbohydrate, 4 calories per gram of protein, 7 calories per gram of alcohol, and 9 calories per gram of fat. What you want to do is multiply the number of grams of each by 4, 4, 7, or 9. That will tell you how many calories of carb, protein, alcohol, and fat you are getting.
Note that with carbs, there is a lot of discussion about how to count fiber. Some say it counts, some say it doesn't, etc.. You can google for "carbs fiber" to find out more details.
For question 2, I've noticed that the food labels are often not precise, and so your own calculations may not match what the food label says. I have read, for example, that if a food contains 5 or more grams of dietary fiber, that fiber may be subtracted from the total calories at the discretion of the manufacturer (who typically won't tell you whether they did or didn't). Also gram values are often rounded. In addition, your body will convert the food into energy more or less efficiently than someone else, so the number of calories of energy you actually get will be different. It really is a game of us consumers trying to get to the truth versus the manufacturers trying to sell a product ...
Ok - I went off on a little bit of a soap box there. Sorry. Anyway, I hope at least some of this helps.
Brian.
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