One person stopped by while I was setting up my Crown Graphic and he showed me an app on his iPhone that can give you an exposure using the iPhone's camera. Granted, I don't know how accurate that app is. I also asked him, how accurate it was and I don't think he had relied on it for his exposures.
Personally, I prefer an analog meter over a iPhone app.
And you do good in doing so. This iPhone App usually is at least one stop off. Sadly one can never tell wether it is one too bright or too dark this time.