Ah! I take it this is (at least partly) why providing some detail about default sleep tolerances is so complicated. I totally see why it would be hard, if not impossible, to assign the default tolerance a numeric value or range.
With that, I'm thinking that what I'd actually like is closer to quality of service rather than a numeric value. That way I'd be able to make an informed decision about how my tolerance requirements compare to the effort the clock will make to wake my code up at the proper time, and then either give the clock as much leeway as it would like, or instead pass my requirements on to the clock.