Am I counting calories in our meals right?

Counting calories can be tricky for a homemade recipe. I think I've got it figured out (to at least give me a ballpark estimate), but when I did it tonight, the calories in what I ate seemed terribly low.

I took the individual ingredients' weights and looked up the calories for those amounts. Then I added up how much the whole dish should weigh based on what the ingredients weighed. Then, when I ate, I weighed how much my portion was. Then I figured out the calories per ounce for the whole dish and used that to determine the calories I ate.

I'm not expecting this to be right on the money, but I'd expect that I'd get reasonably close, and if anything over-estimate my intake, which is the preferable direction for error I say. Am I maybe doing something seriously wrong with my logic here? Just checking.

An example:

The dish I make has 4 ingredients:

Ingredient 1 weighs 1 pound and has 500 calories

Ingredient 2 weighs 2 pounds and has 800 calories

Ingredient 3 weighs 0.5 pounds and has 200 calories

Ingredient 4 weighs 1 pound and has 200 calories

The final, combined dish should weigh around 3.5 pounds and have 1700 calories in it. That's 30.4 calories per ounce, so if I have a half-pound serving, I've consumed around 243 calories.

Yes? No?