Usually powders are used at 1 TBSP per lb, otherwise the soap can be too scratchy.
In my opinion though, if you want to use it for it's medicinal qualities, comfrey is better suited for a leave on product instead of one that gets washed away right away. Comfrey needs to sit on the skin to be effective.
Also, if you're making anything with comfrey in it for sale, keep in mind that the herb is banned from products in some countries.