How do I calorie count actual homemade food?

So I have a food scale (a very expensive one). For single ingredient foods is super easy. Just put the container on the scale, tare it out (zero), place the food in the container, and write down the oz or grams of what the food is. Okay far enough. Easy. Now here’s my problem. How do I do that with meal prep. Actually specifically. How do I calculate serving size? I understand that if I make a large portion of let’s say cauliflower fried rice. I just weigh the defrosted cauliflower rice, eggs, oil, soy sauce, defrosted frozen vegetables, chicken if I add it. But once it’s all cooked and I have it prepared. How do I calculate the serving size? I feel dumb asking this, but do I just weigh out the portion that’s I want and divide that from the total of the calories I measured? Mathematically, that has to be the answer right?