You subtract the percentage of pine tar you will use from your oil amount .I can't remember which calculator I used for the pine tar SAP . I have made 10 and 20% batches . I would go with at least 10% . I don't scent it , you could , there are some suggestions on the miller soap site for scents that go with it . I like the smokey scent the way it is , but you don't smell like you have been standing downwind of the bonfire after you use the soap.
Kitn
edited for clarity