I agree with Oleg's point: your KPI should be traffic, not SERPs. Over time your traffic data will give you a very clear read on how well your article is performing organically.
With that said, we're antsy SEOs and when we drop from number 7 to number 22, it's pretty hard to sit there and wait calmly for the next Moz report to come out next week. This isn't scalable, but when I see keywords that I really care about performing in a surprising way, I like to go throw them in this free tool: http://www.whatsmyserp.com/serpcheck.php.
Refresh the results a few times and you'll get a sense of the range of your SERP, rather than just a single snapshot. Google seems to be moving things around constantly, so the single snapshot may not make the most sense anymore. I wonder if Moz is thinking about taking several snapshots and averaging them -- or doing a rolling average over time -- to level out some of the volatility we now see.
Or we can all learn to be more patient and take SERPs with a grain of salt!
