Alerting based upon the Ratio of two data points

[rizwank]

I've got two values, say

Active Licenses and Total Licenses.

I'd like to have an alert setup, where if Active Licenses/Total Licenses > .8, I get an email. I can't figure out how to put that short of math into the Alert UI however, it denies any attempt that I make.

Is there a good way of doing this?

Thanks,