sounds like you actually have humidity where you live. I think you will do fine with the duck eggs running like you have been your chicken eggs, it sounds like the biggest thing that will affect them now will be either breeder nutrition or fertility rates, those 2 things are out of your control when you do not tend to the flock where the eggs came from.Usually when I do dry where I live it's at 30-40% I did add a little bit of distilled water to the incubator just a little bit like 20 ml of water nothing big. Just to get it started up. But I will let that water evaporate and I won't put anymore unless it gets too low.
My last chicken hatch I had it at 18% the entire time and I had a 90% hatch rate.
I love incubating duck eggs because they are SO much fun to candle! that was one of the biggest reasons that we donated duck eggs to the school project, so that the kids could SEE what was going on in there.