The max topical usage rate for spearmint is 1.7% if that helps. I suppose you could mix it with a bit of peppermint or eucalyptus to stretch it out, since they have higher usage rates.
I recommend an affordable book called "Essential Oil Safety Files by Lea Jacobson". It'll save you a lot of money rather than buying the tisserand book. The tisserand book is in-depth and alot of reading, but the lea jacobson book is basically just straightforward easy to read usage rates of every essential oil, up to date. I think she's an American aromatherapist.