While going by calories appeals to my mind as well, being an engineer, I do see that as a bit of over-thinking.
It may be better to go by cans/meal and meals per day.
While calories are the fundamental use of food, there's other considerations that will take priority (hunger, nutrients, etc).
A big tub of multi-vitamins will take care of some of that, but you still need to figure out things based on food groups, calorie-density, etc.
I try to divide mine up into complete "meals" for my family of four:
* one pound meat, two cans tomatoes, two cans beans and two cups rice is a pretty generous meal
* one pound meat, one jar sauce, one pound of pasta (again, a big meal)
* one pound meat, one jar sauce, two cups of rice
* 6 packets of oatmeal, 6 eggs
How many meals we're going to need per day (2-4) is going to depend on what kind of activity we're doing, how much time we have, as well as what level of hunger we're able to stand.
When SHTF you're going to have to decide whether you're staying on full-rations, going to half-rations, or even double-rations to get through that scenario.
Maybe some days you get only one huge meal while others you're snacking constantly through the day.
Bottom-line, your energy requirements are just one aspect of food. Being able to gradually get your family down to 1200 calories/day is a lot more difficult than just announcing it at a family meeting, especially if you're going to expect them to be doing physical labor that they're not accustomed to. Nobody is expecting scurvy to set in after a couple weeks, but if rice/beans/meat/cans is all you've got things are going to get desperate REALLY quick, if only for a chocolate bar and a fruit cup.