I'd be more likely to go with the fixed-priority-backup-job option.  You should
be able to do that as an SM user.  If the top of your C queue is at 152, and
the backup job number is, say #J1024:

  :ALTPROC JOB=#J1024;PRI=160

That way, light-usage sessions will still get all they need and the folks who
are running CPU-intensive stuff won't stop backup cold in its tracks.  Another
option is to get WorkGroup Manager, put backup in its own group and give it a
guaranteed 10% of the CPU.

Ted
--
Ted Ashton ([log in to unmask]), Info Sys, Southern Adventist University
          ==========================================================
A scientist can hardly meet with anything more undesirable than to have the
foundations give way just as the work is finished. I was put in this
position by a letter from Mr. Bertrand Russell when the work was nearly
through the press.
                                        -- Frege, Gottlob (1848 - 1925)
          ==========================================================
         Deep thoughts to be found at http://www.southern.edu/~ashted