It does depend to some degree on the brand, but if winging it I assume an oz per 8 oz of milk if reconstituting as milk. So if, eg, you were adding cm as the lye liquid at the 50% rate and wanted to add the equivalent of 8 oz worth of actual/liquid milk, I'd put in half an oz. I hope this makes sense.
I mix it with enough hot water to make a thick slurry, even then it can be a pain to mix in. If needed, I use the stick blender to get it smooth, then I mix it into my oils before adding the lye. Make sure any water you use is deducted from your recipe total.
They have lots of stuff I can't get locally, and a flat $5.00 shipping that can't be beat.
Here's another site's mixing directions-
Suggested Use: To replace a glass of cow's milk mix 1 - 2 tablespoons (6g - 12g) of coconut milk powder with water. Coconut milk powder is also delicious when mixed with juice, yogurt, or added to your favorite smoothie.
To make "light coconut milk" mix 1/2 cup (40g) of coconut milk powder with 1 cup warm water.
To make "coconut cream" mix 1.5 cups (120g) coconut milk powder with 1 cup of warm water.