When tracking food in that way, it's actually pretty impossible to be accurate. If you are trying to be extremely strict, the best way is to weigh all your ingredients prior to prep, weigh the total casserole in grams or ounces and then calculate your serving by weight. I know many people who do that, especially if they have one recipe they make over and over again. I've never had the patience for it but I pretty much know that trying to eyeball out a serving is probably not going to be that accurate. Also overestimate to be sure you to overeat whatever you measurement is (carbs, calories, etc).
I have a chili I make about once a week that makes "about 10 servings." I ended up one day painfully weighing out everything so that I can get a gram measurement.