Chris, it’s all good advice above. Keep it simple!
I don’t know of such a precise calculator, but to try to answer your question we can do some simple calculations to work out how much you’re burning in kJ/hr or kcal/hr based on your power over time.
However, you’ll quickly realise that you can’t take fuel in at the same rate as you expend it…90g of carbohydrate per hour is ~360kcal per hour (1g of CHO has 17.2kJ or 4.1kcal energy content). You’re probably expending at least 500kcal/hr. If I average 167W for an hour, that’s ~600kcal (you can find the calculation here in the TR Blog).
Don’t forget you’ve got a plenty of fat stored up to use, not to mention ~400g of carbohydrates sitting in your liver and muscles waiting to be used up!
For longer rides, take in what you can stomach! You will need to experiment.
See also here in the blog: https://blog.trainerroad.com/are-you-eating-too-much-here-are-five-situations-you-might-be/
Good luck with your event!