The book is Feed Zone Portables. Yes, it’s a cookbook, but the first 50 pages go through the detail you are looking for. I’ll run through what that would look like for me.
I’m 6’3" 180lbs. 82kg. 20%bf
In the charts, for my weight and and body fat I would have 1400 glycogen calories if I were on a moderate carb diet (40-50%), or 2400 glycogen calories stored on a high carb diet (60-70%). I’ll use the high carb number for this example. And this is just lower body glycogen.
Another chart list data for performing a century. Let’s say I do a century in 4.5hrs averaging 285w. That would be around an IF of 0.75. So slightly assuming 75% of energy use is coming from carbs and 25% from fat, we can calculate the following:
Total calories needed: 5500
Calories burned from carbs: 4125
Calories burned from fats: 1375
Glycogen onboard in muscles: 2400
Glucose in liver: 400
This gives us 4125 - 2400 - 400 = 1325 additional carb calories to intake over 4.5hrs just to finish up even. That is 294 carb calories per hour. And that needs to start from the beginning of the ride.
So if you know your power and duration. And you can come up with close enough number for what percentage of burn has to be carbohydrates. You can work backwards for any ride whether it be a 4hr ride, or even 1hr trainer workouts to figure out what you need. And this is what they detail within those 50 pages. They even discus the same thing for hydration. Most of the time for activities 3-6hrs, if we replace half the calories we burn per hour, with carbs, we will come out pretty close to the calculations.