As TriNick wrote, I wish it was that simple to take 70.3 finish time (or pace) X 2 = Full finish time. A ‘half’ is not really half of an Iron. In distance, yes. Nothing else though.
It is. There are others here more adept at calorie calculations than I, so I’ll defer. Regardless of the equation presented, the basic premise is to eat a lot because it fuels the marathon. And also as TriNick wrote, halfs usually don’t have special needs bags, and I’ll add that you are not required to use a special needs bag in a full. In fact, I believe that it’s just the North American events that have them, or at least not every 140.6 uses them, FWIW. My point being you should be able to go the full 112 with 1) what you have on you and 2) what they ‘hand up.’ You should absolutely not count on the special needs bag being there for you. If it’s there, great. If not, meh. That’s about as much confidence and reliance you should have on your special needs bag.
It could very well be a disastrous decision to only carry half of your caloric needs and count on the other half coming from your special needs bag. The list is long: It may not make it from bike drop off to the special needs area. It’s never really 56 miles into it, so you 50%/50% calorie count will be off. The volunteers may not find it (or take forever to find it). What you put in there may ‘go bad.’ What you put in there may fall out. You probably won’t get anything back. You have to slow down/stop to get it. You have to take time to take crap out of it. My point being: Not worth it.