Calories or carbs (grams) per hour target is a good way to figure out what and when to eat during a race.
What I'm not sure about here is why to subtract the first 90 minutes..? Sure, your body has stored glycogen for appr 90 minutes from pre race carb loading, if this is what you meant. But since you will anyway be on calorie deficit towards the end of the race, and you are limited by the maximum rate at which your gut can absorb carbs/energy, why not fuel at your maximum rate from the beginning?
My advice would be that learn what your maximum carb intake per hour is and fine tune your fueling strategy accordingly. Most people can handle at least 60 grams per hour, but you may find that you still benefit (w/o gut issues) by raising that towards 100 g/h or even slightly above. Note that in theory, with a mix of glucose and fructose (with ratio of appr. 2:1 - 1:1) you should be able to absorb higher amounts of carbs compared to only glucose. And yes, this takes some practice. But in the end of the day, it really isn't that complicated as it may first sound.
What I'm not sure about here is why to subtract the first 90 minutes..? Sure, your body has stored glycogen for appr 90 minutes from pre race carb loading, if this is what you meant. But since you will anyway be on calorie deficit towards the end of the race, and you are limited by the maximum rate at which your gut can absorb carbs/energy, why not fuel at your maximum rate from the beginning?
My advice would be that learn what your maximum carb intake per hour is and fine tune your fueling strategy accordingly. Most people can handle at least 60 grams per hour, but you may find that you still benefit (w/o gut issues) by raising that towards 100 g/h or even slightly above. Note that in theory, with a mix of glucose and fructose (with ratio of appr. 2:1 - 1:1) you should be able to absorb higher amounts of carbs compared to only glucose. And yes, this takes some practice. But in the end of the day, it really isn't that complicated as it may first sound.