I'd like to bouce this off you all to make sure that I'm understanding
things correctly.

Lets say that my total data to be backed up is 83.5GB, and from some
initial trial runs, I've achieved at least 50% compression, bringing that
down to 41.75GB.

I've set my dumpcycle to 1 week, and runs per cycle to 5 so that the
backup runs only on weekdays (when I'm available to deal with possible
errors, etc.).

Since my tapes are 20/40GB, given the compression that I'm getting, I
should only need 3 tapes for the entire dump cycle, correct?  2 tapes to
handle the dump cycle, and one extra for security.

If I'm understanding how Amanda works, during the course of the dump
cycle, she performs a full backup of each disk in the disklist.  She uses
her keen wisdom in determining on which days she does a full backup of a
particular disk, and on which days she does an incremental; nonetheless,
by the end of the dump cycle, she will have consumed a total of 41.75GB of
tape.

Does this all sound right to you?

Thanks,
Stefan