Formulas are pretty much useless for this. You can't replenish during the ride what you burn during the ride- you will get sick. So you need to learn how much you can eat. 250 cal/hr is a good start for most people. It'll change with changing conditions- heat and dehydration both make it easier to feel ill.

The more trained you are the better you get at burning fat. That means it takes longer to bonk. You bonk because while the body can run on fat, albeit at a lower power output, the brain only runs on glycogen. When your blood sugar runs low you get slow and stupid. It won't hurt you but it's not pleasant. If you do it enough times you will learn to recognize when the bonk is coming on and will be better able to function while bonked.