If your buddy is running them the same as chickens in your part of the country and getting good results, then that's what you should do too. It is a highly regional question and no advice you can read on-line is as valuable as the local experience of other hatchers.
The reason humidity matters during the first 25 days is that the ducks need air in the air cell during the hatch. A higher humidity causes the air cells to develop poorly because the moisture can't evaporate out of the eggs. When the eggs get to hatch time, they poke their little bills into the air cell and try to breathe. If it's too small, they either can't keep their bills out of the wet membrane and they drown, or they simply suffocate before they've built up the strength to pip the outer shell. If the air cell is very tiny, they can't even reach it to poke through and they drown that way. I have had this happen very recently and it's very sad.
If the humidity is too low (not something I've ever had happen but then I live in the humid Southeast), then the air cell develops too quickly and the duck runs out of room to develop before it's ready to poke into the air cell. It dehydrates and dies. (I understand this part from reading, not experience).
If your humidity is right, the air cell will fill 1/3 of the egg at hatch time, and the ducks will hatch beautifully.
The reason your location matters is that while the humidity of the air *around* the incubator affects the air cells too. If the air coming into the incubator is humid, then the heat of the incubator will evaporate that moisture first, before evaporating it out of the eggs, in order to reach whatever humidity you are trying to maintain in there. If the air coming into the incubator is dry, then the heat has to evaporate more moisture out of the eggs in order to attain the humidity you're trying to reach.
So if you're in a dry part of the country, you'll need more moisture in the incubator to reduce the amount that is being evaporated out of the eggs; if you're in a wet part of the country, you'll need less moisture in the incubator so that more is pulled from the eggs.
Which brings us to misting. Misting is a helpful tactic for increasing air cell size also. The moisture directly on the shells attracts moisture from inside the egg (I think it's the polarity of the molecules that does it, but don't quote me on that) and causes it to evaporate faster. So if you're having a hard time, due to high ambient humidity, in getting the air cells to develop, then misting is a great practice.
All of which is a very complicated way to say my main point, already mentioned, which is that if you know someone who hatches ducks successfully in your climate, then do what he does and you'll do great.