How do you replace buttermilk with powdered buttermilk in a recepie?

I add the buttermilk powder to the dry ingredients and add water to the wet ingredients. The brand I have, SACO says for 1 cup buttermilk use 4 TBSP blend with one cup water.

Use the proportions on the package -- say, if you want 1 cup buttermilk liquid, follow the directions to make 1 cup (powder + water).