Still, you should buy when it starts going back up, NOT during it going down.
A don't know about bitcoin, but buying any asset when it is going down is a sure path to ruin..you just never know when it is going to stop.
Well, if you mechanically DCA, maybe OK, especially for indexes, but then you shouldn't increase or decrease your DCA-ing based on your price perception.
The latter could be erroneous.