bottom picking is quite an art... I like to follow the VIX pattern going thru bull/bear cycles.... you can see that VIX is like a ball bouncing off a floor.... it spikes really high on some outlier events (e.g. the 07-08 crisis), then every bounce goes lower and lower, it peaks at 50-40-30-25-20 etc etc... nowadays 20 is a tradeable bottom.
again, no exact science here.... buying at 20 could very well be catching a knife... but watching VIX levels, combined with watching previous pivot points, combined with watching other sectors/countries, you get a sense of if any drop is a pull back in a bull, or a thrust in a bear.