It's really a personalized science, ratios of EO's. For example, I find that three drops of basil damn near exfoliates my scalp while two drops puts me in nirvana. Three drops of lavender is just perfectly perfect but if I do two lavender and one basil it's too much.
Each EO has it's own properties and each person reacts differently do it.
My personal advice is to start with(in your baking soda OR your vinegar rinse) two drops of one type of EO. The next time you wash try two drops of another EO. Get to know them as individuals and then mix them to see how they like one another.
Something to also bear in mind-- EO's are very concentrated. They're powerful. Too much of a good thing can cause upset but too little of something can also not be as helpful as you need it. Like I said, it's a personalized science.