I figure 250 cal./hr for elapsed time, not riding time. That's because that figure is supposed to be about what you can pass across the stomach wall, not what you need. When you're stopped you can process a little better than when you're moving. With training, some people to about 400 cal./hr., but I find the 250 to be about right for me. As unterhausen says, it's important to keep it coming in small quantities and never to eat too much at one time. Even so, if one pushes a bit too hard the digestive system can shut down, which can lead to a bonk, even if your stomach is full. The cure for this is to stop eating. Start pushing plain water and taking Endurolytes, at least one every 1/2 hr. This procedure has fixed the problem for me and everyone I've used it on.

I also find it easier to have about 1/2 my calories from liquid food on a ride of over 300k. On shorter rides, I get an even higher percentage from liquid food as effort will be higher, thus digestion worse. A friend of mine took a suitcase full of Ensure to PBP, since it doesn't seem to be available in France. He used drop bags to distribute it over the course.