How much essential oil to add to shaving soap?

I've made soap from this recipe:



http://ift.tt/1i1mBpI



and it is very smooth and lathers very well!! But - I want more...



I've read about putting menthol, tobacco, peppermint and other scents to soap.



As I have several 'chunks' of this soap to use in the future, I'd like to make them different for each use. I melt them with a couple of 10-15 second shots in the microwave and I figured that I should add the scent at that time.



Question is: How much to add??



Thanks in advance to the experienced...



Phil