Nutrition accurate for recipe creation?

I am curious about the accuracy of the recipes I create in MFP. For example, I recently made spaghetti sauce to serve with turkey meatballs. For the sauce, I put canned whole tomatoes in a food mill, sweated minced onion and carrot, then simmered both until reduced sufficiently.
Each one had lots of nutrition, but doesn’t that change when cooked? How do I account for that in my daily diary?