I've used maltodextrin for about 8 years on randonneuring rides over 200km, in addition to solid food at occasional stops. I have no idea how many calories per hours, and honestly don't have the energy to find the answer (pun intended). About 1/4 cup of pure maltodextrin in a large bottle is what I've settled on, and I drink about 1 every 2 hours.
I went to England this year for LEL and decided for some stupid reason to abandon the malto in favor of foraging for fuel en route. I DNF'd for the first time in 12 years of rando and ultradistance riding. There were other issues for sure - saddle and sleep - but poor fueling led to the vicious spiral that led to DNF.
Pertinent point being, I think, that you have to learn what works for you and stick with it. At least don't change things before a big event, which everyone (including me supposedly) knows.