I usually add 1-2 capfuls to a 16 oz bottle of whatever. One and a half oz. seems like a lot, but if it makes your hair feel good go for it.
I really dont think you can add too much and cause harm to your hair, but I'm thinking too much is just wasting product.
So I really don't have an answer, but they usually give percentages of how much to add to your formulations on the website you buy from. HTH.