What is the optimal per cent utilization for a tape backup pool, when
collocation is turned on?

We use 3590 tapes (10GB capacity, uncompressed).  On one server when we do
a
QUERY STG command, it shows 37% utilitzation for backup tapes.  On another
server 52%, and on a third server 37%.  Our average amount of data backed
up
per client is about 4GB.  Management says we're paying for too much tape
space that we're not using.  But we're also concerned to keep restore times
down to a minimum by not having client data distributed across more tapes
than necessary.

Does anyone have a good rule of thumb (or any kind of a rule at all) for
optimal tape utilization with collocation turned on?

Thanks,
   Alan