There was a time, before Pyrex and Oxo, calculators and even cookbooks, when rules of thumb ruled the kitchen. My mother taught me my first one when I was six and still standing on a barstool to reach the kitchen faucet, the infamous and eerily accurate "one-knuckle" rule for cooking rice. Like all good R.O.T., the measures were flexible. It didn't matter how much rice or what size pot or what kind of stove. It worked.
Through the years, others joined my mental chart of measures. Start frying spring rolls when a dry chopstick, lowered into the oil, sends up a rush of bubbles. Don't eat at a restaurant with the word "authentic" translated into English. A thin girl eats one bowl of rice, a polite girl two bowls. The brownies are ready when only a few crumbs stick to the toothpick.
Cooking school and restaurant work lengthened the list of informal guidelines. Cook a fish ten minutes for every inch of thickness. A properly butchered beef carcass will produce 1/4 steaks, 1/4 ground beef and stew meat, 1/4 roasts, and 1/4 waste. A classic mire poix is 2 parts onion, 1 part carrot, 1 part celery. Roux is equal parts flour and butter by weight. A cake is done when it springs back from a loving tap.
Professional caterers employ some of the most specific calculations for feeding people that are, in their way, merely interconnected rules of thumb. Are the guests standing or sitting? Sipping lemonade or knocking back cocktails? Celebrating a friend's birthday or waiting around to sign big, fat donation checks? Are the plates 5" or 7" wide? Are there mostly women or men? Is it a cold morning or a hot night? Each factor changes what kind and -- more importantly, how much -- food appears on the tables. In the end, it's as much a guesstimate as that thumb in the rice: based on experience, fluid, and as exactly right as it needs to be.