If you think it's silly to get caught up in probabilities, then why would you say 10% even? Being an atheist is about rejecting the idea of something existing that leaves no proof as being highly improbable. Look up Russell's Teapot; Bertrand Russell said we can never prove there isn't a teapot, too small to be seen with our most powerful telescopes, in orbit between the Earth and Mars, but almost everyone would agree that the probability there is one is virtually 0. Your argument would indicate it would be meaningless to assign probabilities to Russell's ridiculous hypothesis too, since it can't be shown one way or the other. I guess by God I would mean the active god who creates a universe, watches every person, knows what everyone is thinking, etc.