Most people just sprinkle it on the ducklings feed for the day. Not exactly sure how much that is when they say sprinkle.
I use about 2 tablespoons per day for my older birds (split up between 11 ducks and two geese now) and I used about 1 tsp per day for the ducklings. One reason I used so little is that I let my ducklings (and now older birds) eat waterfowl feed. The waterfowl feed already has a higher amount of the niacin in it. I was fortunate to find 2 different waterfowl feeds at my feed store. The nice thing about the brewer's yeast is that it has a lot of different nutrients and as long as your not using crazy amounts of it then it is hard to overdose them on it, making it safer then using crushed pills of pure vitamins. However if your using chick feed it will have less niacin then the waterfowl feed and you may want to use more then I did.
Not sure if that really helps you know how much to use or not but I will say that I didn't do it for my first batch and I noticed a difference in all my ducks as soon as I started using it. They seem better on it. My third batch I added it to their feed from the start and I will say they did look better. Standing straighter and running better.
You could try asking it the duck forum. Perhaps other can give a better answer on how much...