For those who don't know: since unstable rate is still quite an obscure measurement, here is some very detailed explanation from Full Tablet about it. Check out his Excel table here, it sums everything up. It's still a hypothesis but it looks quite good.
(tl;dr: it says how accurate you are depending on your hit precision. Depends on number of circles, accuracy you got and OD. The lower the better)
I think this is my best one so far, not sure, though. I think if I hadn't missed the one note, and I had gotten my SS, that it would have been about 60ms. It is the one and only time I ever paid attention to my unstable rate, I never usually check it.