here you go, there are several links on a search--looks like 1/4 teaspoon for 1 teaspoon-
Questions and Answers - Peppermint Oil vs Peppermint Extract
Peppermint extract - Ingredients, Substitutions and Equivalents
and this is a neat site for checking lots of things-
Cook's Thesaurus: Extracts and Flavoring Oils
I am a bit concerned, are you sure this oil is food quality grade? It should say on the bottle or you could give the health food store a call. I just seem to recall having some essential oils that I had being labeled "not food quality grade"?
sounds like a nice recipe. I tossed the catalog, but I think it's on Williams Sonoma website--white chocolate bark covered with crushed peppermint candy--thinking there must be a fun way to use this concept.
Have a super day,
edit---was posting at same time as jfain-whoops, she said it better and way shorter!!!