Yes there is the science of what your body can take in, but be careful as these are numbers from small groups of people who are being studied. You might need more or do ok on less. It's a starting point. On my last big ride (struggle dales 2021) I had a real spread of foods including honey sandwiches - as benjones800 said. Real food starts to make the difference for keeping feeding and I was looking forward to the sandwiches too.