Portioning is part of the art and science of catering. If there are hard and fast rules, they are so riddled with exceptions and irregularities that they come with an ironclad guarantee that whatever you're doing doesn't fit within them.
The smallest eaters are older people, women when men are present, children younger than high-school age, and so on.
Women in an all female gathering, eat more than in a mixed group. People in active occupations eat more than desk workers.
High school and college men will eat to impress. If the gathering is based around any physical activity -- even as spectators, they will eat AND drink more. And so on. If you're doing a tailgater, with a ton of filling apps -- a pound of steak per person is begging to run out.
If a man eats 1 portion -- a woman at a mixed event will eat about 70% of a portion, and at a hen event, about 90%. But not always. I used to take turns catering a once a month lunch for 8 to 12 women in the entertainment industry -- all of whom were always on a diet, except for that monthly lunch. Eat? Eat!!! About 1.75 servings per woman.
Plated events get a lower per person allowance than buffets, because chaffers must be kept full with fresh food. Family style -- which by the way is a very good thing to suggest -- a lot of fun for the guests, falls in between. "Silver service" is the smallest allowance per item, but usually involves a complicated menu.
Evenings usually mean more per person than days, but a day time buffet organized around an athletic activity -- whether spectator (tailgate party, for instance) or participant (hunt club, for instance) are the second biggest eaters -- tied with things like fire department dinners.
Church dinners -- big eaters.
Other charity dinners -- small eaters
Athletic team -- big eaters
At a mixed buffet:
A woman's portion of 4 oz, a man's 6 oz, before cooking.
A woman's serving of chicken is two pieces, counting the wings as half pieces, a man's is three.
A woman's serving of ground meat is 4 oz, a man's 6 oz
A petite steak for a woman is 6 oz, a small steak for a man is 10 oz. If you serve a 40 year old man an 8 oz steak, he'll say thanks and smile, but he'll feel deprived. A 12 oz steak is "more like it."
And so on
When food is served plated, there is no separate allowance for men or women -- you have to portion for a male reasonable eater. But you don't need much allowance for "seconds."
Some portion sizes are determined by the piece of food, particularly if bone is involved. For instance:
One 15 pound, whole, uncooked turkey will barely feed 12 people; while a 22 pound turkey will feed 20 easily.
One slab of spare ribs, withhout tips attached will feed 3 people, with tips about 3.5.
One slab of baby backs will feed 2.5 people.
One whole 8 oz trout per person.
Depending on the event, you can get 2 or 3 per person per bone of bone-in prime rib. You cannot get less than 3. However, you can get more than 3 if the same roast is boned out and the bones served separately. Don't ask why, you just can.
And so on.
Every one of the rules I wrote is riddled with exceptions -- just like I said on the top of the page.
The more you do this, the better sense you'll get of how to determine waste from raw and untrimmed, how much to allow as a cooked portion, what different types of sauces mean, what multiple courses mean, how to calculate shell on shellfish, and ... well there are hundreds of other variables.
I'm in no position to criticize because I don't know all the facts. Saffron's protein estimates seem very low to me, except as part of multiple course service -- for instance a "tasting" menus. But she may routinely provide a varied and satisfying group of sides, appetizers and specialty desserts which would change everything. Actually, no "but." I know "lots of little" is how she thinks. As I said, general rules aren't really helpful.
When you wandered in here, you hit a real treasure trove of people who can really help you with this. Chef Ed is especially knowledgeable about really large groups. Shroomgirl has the broadest knowledge base for catering I've every seen. In fact, I can't think of anyone who contributes to the catering questions who I'd say wasn't worth listening to closely.
Here are four rules that are always true:
People don't hire a caterer to run out of food.
People don't hire a caterer so their guests, too polite to take the last pieces, leave hungry
Better a little too much than a lot too much, but better a lot too much than even one serving too little.
Be prepared to package leftovers attractively for guests. That's a much better sign of success than "perfect" portioning.
Just a few thoughts,