It is a fine line to walk, that's for sure! You're probably right...85% is too much. I have a clear tube from the outside of the bator into the water dish. If it gets too high, I suck water out with a syringe. I don't use a sponge for this reason. I have more control.
After they pip, you just have to know that you have enough humidity to keep the membranes inside the egg from drying too much and sticking to the chick.
The last chick that hatched from my last bunch had to be helped out. It had pipped but couldn't seem to move. After many hours, I opened the pip hole a bit larger and could see that it looked like it had been saran-wrapped into the shell. I quickly peeled the top of the shell and had to run it under warm water to be able to get the membrane off of the chick. It was stuck so hard I even pulled off a tiny fleck of down from it's wing.
I still think more chicks die of drowning in too much fluid rather than membranes that are too dry.