An average person in a sedentary job (office / lecturing / teacher - unless you have to chase pupils all over the place..) is likely to require 2000 - 2200 kcal/day. Someone with a job requiring greater exertion e.g. a construction worker or soldier would require anywhere from 3000 to 6000kcal/ day.

A 35 year old man weighing 72kg (158lb)with a height of 1.73m (5'8")would have the basal metabolic rate of about 1700kcal. This is the amount of energy your body requires to function for 24h if you don't do anything ( i.e. the energy requied to move your gut, get your heart pumping, breathe etc)

The 1200 kcal mainstay bar is just to reduce as far as possible the catabolic effects of starvation (that is to reduce the rate of protein (muscle) breakdown).
If you were a smaller person, weighing less, you would require less energy as basal energy expenditure, the 1200kcal bar would bring you further.

That being said, your body would burn up your fat reserves first before touching the muscles, so, scientifically speaking, your body doesn't need the mainstay bar in the initial period as it breaks down the glycogen and fat reserves. Therefore the mainstay bar would be most helpful when you have no more reserves left.

That being said, the psychological effect of hunger on the survivor as well as the moral boosting properties of any available food are why most emergency rations are included in lifeboats. If you hope to be rescued in 72 hours, you won't need the food.

In the sea of plenty, our bodies are used to so much food that the moment the amount goes to zero in a survival situation, our bodies cry out for more and headaches, hunger pangs and nausea set in. Once you get over the initial wall, you should be feeling fine.
Just for interest sake, a mickyD's meal of a bigmac (580kcal) + med fires (320kcal)+16oz coke(190) = 1090kcal, (this info is from the McD's in Singapore - the US servings may be different) meets more than half the daily caloric requirement for most people.

So, if you are packing mainstays to meet all your caloric requirements for a long term bug out, 2400kcal per day is more like it. For short term 72 hr situations, you don't need the food. But if you require full functional capacity (e.g. soldier on ops or SAR) pack more. And more variety. If you are just waiting for rescue and for a morale boost, in the 72 hr kit, I'd rather have a couple of snicker and mars bars (280-300 calories each).
Just my humble opinion.

HTH