Calculating Nutrition Information for Commercial Food Labeling
Use Wolfram Alpha, it is just adding them up. But any lab result has to use average ingredient figures too. You need to allow for variance in supply i.e protein level of flour changes with variety and season.

It produces pretty labels, all ready to go!

e.g. http://www.wolframalpha.com/input/?i=100g+flour+and+100g+butter+and+50g+sugar