The more I think about shave tests the more I come to the conclusion that its possible to use what ever you want. If your test can correlate somehow to a positive shave test and you have the sensitivity to gauge a grape to a good shave consistently than who am I to tell you that its not valid. If you can teach yourself to make that comparison and it works for you I don't see why not. Its your test its your shave. Tomatoes, Grapes, arm hair, hanging hair, Potatoes what have you. Nothing to freak out about here lol. Experiment have fun enjoy.