I calculate based on a few factors
1. When two meats 1/4 lb. per person
2. A hotel pan serves 50
When you get over two meats and three side dishes you need to decrease the amounts for each item 10% at least. Keep records, the more you do the more tuned in you get. I've taken my share of stuff home, given it to my helpers and called the kids to come over and 'shop'. Hate overages but you will tune in as time goes along.