When I butcher a pig I salvage the cheek meat and tongue off of the head. The rest of the head goes to the dogs since I shoot them in the head and I don't want to bother with picking around all the bone shards. The dogs also get the hide and the feet as well as the intestines, pancreas and lungs. We eat the skeletal meat, heart and solid organs. The bones are pressure cooked down into concentrated stock which we then can and save for soups etc. Done this way, you can utilize roughly 60% of the live weight. Skin is surprisingly heavy, but in a survival situation I'd assume you'd fry the skin up as pork rinds, in which case you'd be approaching roughly 80% of live weight. Also note that humans have a higher surface area to mass ratio (more skin as a percentage of the weight) when compared to a pig since pigs are more spherical than people.
Also amputating and keeping a victim alive in the meantime would reduce the amount of salvageable food because it takes far more energy to try and heal a wound than to simply keep breathing. Also the victim would likely die of sepsis or blood loss. In the middle ages without refrigeration people would preserve meat with salt, drying or keeping it submersed in a stew with a thick layer of fat on top. If the stew is heated every day and the cauldron is covered when not in use, most of the time there was little if any wasted by the time the boar or stag was consumed.
... and you were crash landed somewhere inaccessible with a few other people and a handy corpse how many meals could you make out of that corpse and what would they be?
Remember that depending upon your surroudings a person's caloric requirements might be higher than usual. Most of the caloric requirement charts assume modern society (light work and controlled climate). If they are in the arctic they might need 6000 calories a day just to stave off freezing to death and then on top of that they will be doing heavy labor to build a shelter etc.
It's also worth mentioning that in a survivial situation, one would want to dice up the intestines (washed thoroughly prior to dicing) and cook it as tripe soup. As long as there are ways to cook your food, nearly the whole creature can be eaten. In the third world people eat tripe all the time. Our food sensibilities as to what we will and won't eat is really more a matter of culture than biology.