Are you sure that the amount of holding disk space you have,
  minus 50%, is sufficient to hold those level 0's?

Yes.  They dump and fit.  I have 25G of holding space and only dump
10G or so.

  The whats."NEW" is actually quite old.  Note at the top of the
  document it indicates these are "new" as of version 2.3 and about
  40% of the way through the version 2.2 document is appended.
  The section you quote later is from that appended section and
  is, I believe, misleading.

Well, I didn't find anything else in the docs, so something may need
updating.

  My personal experience is that level-0's quite happily get done
  with my reserve of 30 percent and no tape present.  In fact,
  due to a sloppy, lazy admin, the tape magazine has not been
  switched for 2 weeks and last night there were still many level-0
  dumps done to the holding disk.  The reason this works is that there
  is still lots of holding disk space available.

Sure, that agrees with my experience.  But if you have 'reserve 100',
or no statement (defaulting to old behavior), with no tape I would
expect no fulls to be done, but with a tape they will be.

-- 
        Greg Troxel <gdt AT ir.bbn DOT com>