From my own experience of incubating both chicken and duck eggs, duck eggs do not automatically require higher humidity. They need a higher lockdown humidity, yes, but not necessarily a higher humidity all the way through the incubation. My normal humidity figure for incubating chicken eggs is 45%. With ducks it's more like 35-40%. For me, this is what gets them to lockdown having lost the correct amount of moisture. Chicken eggs need to lose approximately 13% by day 18, duck eggs need to lose approximately 16% by day 25 (For Muscovys it would be a few days later of course). As duck eggs tend to be larger than chicken eggs, they tend to lose moisture more slowly (as they have a smaller surface area in relation to their contents by volume, meaning that evaporation through the shell will be slower) so you can see why they might need less humidity than chicken eggs, not more...
I'm not suggesting that you use these figures for your own incubation, just that you read up a bit on humidity and its purpose during incubation, so that you really understand why you're doing one thing or another, rather than just following people's advice without actually knowing WHY you should be doing what they're recommending.
BTW This isn't intended as criticism in any way - I used to be very clueless about humidity and my hatch rates only started going up once I'd taken the trouble to research and read up on it myself and figure out for myself what would be the best thing to do and why. Most people on here are very knowledgeable and helpful, but some of the advice is poor and a very small percentage of it is just plain wrong! If you learn as much as you can by yourself, you'll be better able to pick out the good advice from the bad.