It's probably best to contact the supplier of your powdered goat's milk. Here are the four pieces of information that I've gathered from various sources. :wink:
My supplier: A rule of thumb is about 15gm per large glass.
To reconstitute to standard, mix 1 part goats milk powder to 4 parts distilled water (by weight), and use as whole goats milk. ...
Measure 1/8 cup of powdered goat's milk for every pound of oil or fat in your recipe, and mix in enough water to give the powder a thin, paste-like consistency.
If you are using your own recipe, hydrate a concentrated portion and add at trace (If using this method we suggest trying 25g per 1kg of Oils).