Check with the vendor of the milk you're buying to find out the concentration. Basically, you want to add in enough powdered goat's milk to make it the same amount of milk as if you were using fresh goat's milk. The powdered goat's milk I use is pretty concentrated. I use about 1 oz. (by weight) of powdered milk for every 8 oz. of water in my recipe.
As you are measuring out your oils, separate two or three ounces of oil and mix the milk into it with a whisk. Stir briskly to get out all the lumps. Set it aside to add at trace.