Another milestone in our first batches of wine!

We've reached the bottling stage. My wife wants to sweeten up her apple and
blueberry wines a bit.

White grape for apple, red grape (concentrates) for blueberry.

She says she needs to stabilize them so yeasties won't take off again. (this
stuff has aged 6-8 months with no bubbles already)

She has read sodium benzoate can be used, we have powder, not a tablet like
her book states.
Can someone please be so kind as to tell me what % solution, or how many
mg/gallon to add?

Thanks

Mark