No, because that doesn't make sense. It may be -0.5 now, but what if it carries on falling to -1, or -5 ? What if the market becomes very volatile for a long time and -3 becomes the new 'extreme' ? If you look on the chart above from Aug to Dec the 'extreme' was only -0.1 that whole time. At what point then was it considered overbought/oversold ?

It's not a bounded range, it's an absolute value with no limits that changes all the time so you can't really quantify it and say 'x level mean oversold'. I think the OP is confused between a downtrend and being oversold. You can still be overbought in a downtrend ! Infact that's exactly what people wait for to enter trades...