substitution guide regular sourdough for dry?

Anybody have advice for me on substituting regular sourdough (mature culture at about 80% hydration) for dry sourdough culture? There's a barley bread formula I'd like to try that calls (in the home baker version) for 1/8 cup dry sour dough culture. By that, they mean the dehydrated product available at specialty food stores. Have never used the stuff, so I have no idea. Thanks!