I just tried your suggestion. The terminology used for this is confusing, and something will probably need to be changed by Symantec.
First, when defining the policy, the "Conditions" screen shows the description for this alert as:
" Action is taken if, available media reduces below preset threshold value."
Second, on the next window that pops up:
"Alert is generated, if available media for the selected server context reduces below this number."
This would make you THINK that you enter the NUMBER OF AVAILABLE MEDIA, but that's not the case. If I try to enter anything above 100, I get an error message"
"Percentage Threshold value must be a positive integer with the range of 0 to 100."
So, it's not asking for a number of media, it's apparently asking for a percentage of media.
I've changed the value to 75. I'm now assuming this means that if at least 75% of the media in the library attached to that server isn't "available" (scratch), then it should alert.
I don't see anything yet. Still don't know how often NOM polls for such info.