Hello Moritz, this is the fragrance calculator I plan to use next time: https://www.brambleberry.com/pages/Fragrance-Calculator.aspx
As far as adding herbs - I haven't do it yet but I know you can add most to soap I would just google around and see what others have used and how they have combined them.
My only tip would be to add your scents and herbs in before medium trace sets in to be sure it mixes well!
Good luck!
RE: Castile Soap (aka olive oil based soap): First try