Making Your Own Iodine Shampoo?

I have a horse with a patch of persistent rain-scald, I've tried using Krudbuster and various other spray on products designed to treat it. Sterilized all of my equipment, followed the usual protocol for dealing with it. After a couple weeks with little improvement I had the vet out and he said to try an iodine shampoo on it. I've checked all of the tack, feed-stores, ect in my area and they don't carry it. I was wondering if it would be alright to just mix iodine with regular horse shampoo? If so, is there a certain formula of iodine to shampoo I need to use to make it effective/safe?