Exactly this. You just weigh every ingredient. It doesn't matter if it's a sauce or what. If it's something premade (like tomato sauce) you use the calories on the packaging. If it's a raw ingredient you look it up.

I never bothered with weighing the final result or portions, instead I just always divvied up the final product into equal individual portions and divided by the total number of portions. That works well if you freeze them.

Of course, all the calculation is a tremendous amount of work. I did it when I needed to lose weight and only did it for a couple of months. But it definitely "calibrated" my understanding of calories -- e.g. non-starchy veggies have barely any at all, while cheese and butter and oil can easily double the calories in a dish.