Usually when the value decrease more than 75 % because this is cryptocurrency not ordinary stock trading.
But whats the difference of crypto from stock trading ? Just because cryptos are too volatile and stock are too stable ? But still in every kinds of investments its always a golden rule to buy at low and sell at high . Its better to wait for more drops so that you can get a bang for your bucks .
Just make sure that the coin were in top 10 on coincapmarket to make a narrow choice / filterring.
Next step is hold for at least 6 month !
Only top 10 coins ? What about others that are on the top 20 , 30 , 40 and so on ? They do also have a potential and ranking does not matter at all imo .