IMHO:
Until the automatic shutoff is approved by your regulating body or company, the best operational tactic is to buy a digital watch with an alarm and work out what time the CT will reach 500kgs. You'll still get a fuel config at TOD most times even if you run it down to 0.3 because of the change in AOA.
The alarm works even if your CTFP eta in the fix page has been deleted by "direct to-s" or if you happen to be in the forward galley shooting the s***.
Just an opinion.