Duck Math is super dangerous! Definitely plan on 20+4!
One tip I would offer when planning your duck coop is to include an area that is separated by hardware cloth or wire etc to use for broodies, duckling brooder, injury recooperation, etc. Even better is if it can have a little pop door leading to its own small run or pen. In the future, every coop we build is going to have that. No more moving dog crates from coop to coop so I can integrate young cockerels into the rooster pasture or young pullets into the Layer group. I will have a much easier time allowing first time broody ducks to raise their babies because they won't flip out at being moved to a different area causing me to have to raise their babies inside in a brooder.