A rule-of-thumb I've found use for in similar situations: There are approximately ten billion people alive, of whom it's a safe conclusion that at least one is having a subjective experience that is completely disconnected from objective reality. There is no way to tell that I'm not that one-in-ten-billion. Thus, I can never be more than one minus one-in-ten-billion sure that my sensory experience is even roughly correlated with reality. Thus, it would require extraordinary circumstances for me to have any reason to worry about any probability of less than one-in-ten-billion magnitude.

There are all sorts of questionable issues with the assumptions and reasoning involved; and yet, it seems roughly as helpful as remembering that I've only got around a 99.997% chance of surviving the next 24 hours, another rule-of-thumb which handily eliminates certain probability-based problems.