>...Should the cache hit rate consistently be above 98%? ...

In a single word: No.  The 99%+ is the fairy tale objective.
You'll find numerous postings in the List archives advising of
the real world behavior of caches which, as you've seen with
Backups, cause the number to drop, as one should expect with a
large number of such unique accesses.  The best we can do is
dedicate as much *real* memory as feasible to maximize the
number during steady-state operations.

Don't get over-absorbed in Cache Hit Ratio: It's just one
factor among the many that a systems person has to address to
maximize server throughput.  Your time needs to go into a lot
of other issues as well.

  Richard Sims, BU