I looked through the recipes I've kept for salt bars and I don't seem to have the one I made with butter. I'm not surprised, as I didn't like the soaps as well as those without butter. I am not sure if I used shea or cocoa butter, which might make a difference. My best guess on the butter usage rate is 10-15%, and probably closer to 10%.
I think you should just try a small batch with 10-15% shea and one without. They take a long time to cure, so it will be awhile before you know what you like best, but it will let you see if the butter adds (or detracts) from what you want in a salt bar.
