Sorry to have left you hanging there.
For me, the show-stopper is the medication. I can add niacin or cut the protein, but I cannot remove the medication.
My feeling about cracked corn is that it isn't as good especially for ducklings as oatmeal. There are some posts (yup, more debate) about whether corn should ever be fed to ducks, and if so, how often, and so on. I avoid the issue by using oatmeal, which is what the Holderreads use when they raise ducks. Since they have a pretty good reputation, I went with their method there.
Niacin can be bought at local pharmacies or big box stores - wherever vitamins are sold. Apparently there is some kind of niacin that is slow release or something and I would avoid anything "special." Or you could pick up brewer's yeast, often found with vitamins, sometimes at health food stores or larger grocery stores (may be in the specialty section).
The amount of niacin to use is difficult to pin down. Storey's Guide to Raising Ducks states that when a duckling shows signs of niacin deficiency, immediately add 100 to 150 mg of niacin per galllon of water until they are 8 weeks old.
But what if we are just trying to head off a deficiency? I wish I could give you the answer. Perhaps somewhere on the forum someone gave some specific wisdom. I know there have been topics on it.
There may also be something else out on the internet with some numbers that are useful.
I also bought some vitamins and electrolytes, which I gave to them once or twice a week the first month, mostly precautionary. There was one duckling who, at about three weeks, was beginning to look puny, so I made sure to give her some.
That may be a way for you to go - add some poultry vitamins to their diet once or twice a week.
