Without getting scientific of pulling out a calculator I tend to assume I'll need one extra major meal per day if I'm touring fully loaded in the mtns. I may eat a bit more each meal, I may snack heavily or I may eat 4 normal sized meals. I don't bother trying to set up a specific plan as each tour is different.

I would suggest you listen to your body and if you feel like you need more food/water make sure you feed the engine. I do carry a lot of snack type food [nuts, jerky, dried fruit, chocolate, fruit, etc...] so I can eat while riding if I want to.