Another attempt to fix doctest.

* Apparently doctests ... don't work the way I thought they did.
  Reduce the length to 3ish digits in all tests to avoid issues.
  The calculations should be better than that.

This may convey the impression in the docs that the computations
are only good to a few digits, which is generally wrong.  But
some values are very close to 'flipping over' a digit, and the
doctest ... specification doesn't really handle that well.