Since it was free I would save it till I need it.
Powder nutrients do not keep well and are sensitive to high temps.
So weigh out your needs, Sure is nice to get something free you could use.
You need a specific gravity meter to tell excatly how much,
I use 4 cups of sugar in a gallon and it works for me, If you have a yeast pack little square guy its good for 5 gallons and 6 months.
Some people add a nutrient to make it faster.