I'm making a recipe and I know the nutrition information for all of the ingredients. If I add each calorie and fat (etc) count together, seemingly, I have the information for the total recipe (with me so far?!). If after it's cooked, I measure how much the recipe made (ie 8 cups) then divide the total nutrition information by whatever a "serving" is going to be (ie 1 cup serving, so total divided by 8) - I should have a fairly good approximation of the counts, right? Somehow this is making sense to me, but it seems too good to be true. Any ideas?!

That's exactly the way to do it. Then you record that information and have it for future reference.