more than 2 fulls on 10 tapes?

I'm sorry if this is a FAQ; I tried searching and didn't find anything.

I am still running 2.4.5p1 because I haven't updated the server side for
krb4 yet.

I have been running amanda for a very long time, and up until now on DDS
(1, 2 and then 3).  Most recently I had 15 tapes, dumpcycle 7 days, and
runspercycle 5 days, so I always had 2 and sometimes 3 fulls on tape.

I just got an LTO-2 drive (no changer), and I am setting up to use 10
tapes.  I'd like to tell amanda

  dumps happen 5 times per week (0005 tuesday to saturday)
  I have 10 tapes.
  I would like a level 0 every 4 tapes.

so that I will essentially always have 2 fulls safely on tape, even when
writing the tape with the next full dump, and even if the full dump is
one run late.

I tried:

dumpcycle 5 days
runspercycle 4
tapecycle 10 tapes

and balance showed:

 due-date  #fs    orig kB     out kB   balance
----------------------------------------------
 7/02 Mon   36   22172589    8364704     -8.2%
 7/03 Tue   25   42117664   20905858   +129.5%
 7/04 Wed    4   14313242    7170225    -21.3%
----------------------------------------------
TOTAL       65   78603495   36440787   9110196
  (estimated 4 runs per dumpcycle)
 (22 filesystems overdue, the most being overdue 2 days)

With dumpcycle set to 6 (and others as above), balance says:

 due-date  #fs    orig kB     out kB   balance
----------------------------------------------
 7/02 Mon   22   11059915    4551488    -50.0%
 7/03 Tue   14   11112674    3813216    -58.1%
 7/04 Wed   25   42117664   20905858   +129.5%
 7/05 Thu    4   14313242    7170225    -21.3%
----------------------------------------------
TOTAL       65   78603495   36440787   9110196
  (estimated 4 runs per dumpcycle)
 (16 filesystems overdue, the most being overdue 1 day)

So, is the scheduler really going to be trying to write a full every 4
runs?  It seems there is some roundoff going on in the above
calculations (one 4- and one 4+).

I think what I'd really like is decoupling of conveying the schedule of
when dumps happen, and the number of fulls to keep on tape, more or
less.  I realize other people want to say "a full every week".

Something like:

operationcycle 7 days
runsperoperationcycle 5
dumpcycle 4 tapes
tapecycle 10

This would mean (avoiding thinking about unflushed dumps in holding
disk) that a full dump is due if there is not one on the most recently
written 3 tapes.

Perhaps I should have just used 12 tapes....

Any clues appreciated.  I will be running 6/4/10 for now.
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