This could be a workable definition, but we'd need to be a bit specific about just who has to be reasonably certain. Your average alien-spaceship believer needs very little in order to be reasonably certain that any given light in the sky is not of natural or manmade origin. Skeptical investigators, on the other hand, tend to have a rather higher standard. This is better, but the problem remains of who is required to evaluate the evidence. Is it enough to be merely "technically capable", or should we require a certain level of actual critical thinking and skepticism in evaluating the evidence? Hynek himself started off his adventures with UFO reports as a fairly level-headed skeptic, but gradually shifted to a certain level of credulity as his base setting, from what I gather.