How much vitamin d should I give my 2 year old duck?

[Heirihwidisbwhhsgxgs]

We recently bought a Jamieson vitamin D3 (400 iu) supplement for my duck. She’s two years old and has a vitamin d deficiency.
We don’t go outside often, but I’ve been opening the curtains for her in the morning for her to get some sunshine. I was wondering how much I should feed her this everyday?

Does anyone know?
Please let me know if possible and note whether you’re certain or not it’s safe. Thank you!

 

[Brooks_]

The vet that diagnosed her didn't give directions?

 

[nuthatched]

Are you sure she's not niacin deficient?
What's her diet?