Honestly it's going to vary from machine to machine, app to app and situation to situation.
For example, MS-SQL automatically allocates 80% of the machine's RAM. It doesn't matter how much you give it. So alerting at 80% across the board is going to wake up (and piss off) your dba's.
Some machines run "hot", or run hot when performing certain operations (weekly full backup! and an antivirus scan! and month end reports! all in the SAME WINDOW).
Setting the thresholds on a machine by machine basis, with a default if that individual setting is blank, may be a more effective way to go.
He is correct in that it will vary from device to device. The best bet is to collect data for some time and then go back and evaluate per device what values have been seen. Then setting threshold per device can become quite useful. Since SW has not provided this feature, another Thwack person made this post about how to do it via custom properties.
Hope it helps
Hey, that's not ANOTHER thwack person, that's ME!!
Seriously, thanks for quoting my stuff.