Given that it is a job interview (and not a trial), that we're talking about a lifetime appointment to the most powerful court in the country, that the Court's legitimacy is absolutely tantamount (and certainly more important than the reputation of any one potential justice), and the difficulty of removing through impeachment a wrongly appointed justice, I would say your doubt doesn't even have to be "reasonable." For example, I think if you only believe there's a 20% chance Kavanaugh assaulted Ford, that may be sufficient to keep him off. Why take the risk when there are plenty in line without the cloud of suspicion that overhangs Kavanaugh?