If there is evidence of niacin deficiency, Storey's Guide recommends 100 to 150 mg per gallon of water for 8 weeks.
If you are just trying to adjust for a lack of niacin in feed, I might cut that by half or more.  So, if you have a steady hand and a good eye, put a quarter of the capsule's contents in a gallon of water.
Now, since I have neither the hand or eye for that, I would make up a batch with one capsule in two or more likely three gallons of water and put the rest in the frij to be added to their waterer when it is time to change it.  I did something like that with the vitamin electrolyte mix.
You might also alternate niacin water with plain water.