I think it is very difficult to guess the low prices in any investment either it is bitcoin price or share prices so the best method to buy is start buying in small quantities when ever you can and get the best average price over the time. I always buy when I get some extra cash and will sell when I make some decent profit.
It is impossible to know what the ultimate low of the price will be. The best we can do is guess and do a bit research as far as that is possible. That's why I always buy when people are unhappy and are selling like there is no tomorrow.
Some times when I buy the price may go down a bit further, but that's not a problem at all as I never go all in. I always do 25% buying at start and leaving 75% fiat in my account for when the price goes down even further. That's how I will continue to benefit from the price going lower. If the price goes down at least $10-$20 lower than when I bought, I use another 10% of my fiat to buy again.
Well you will not do such buying action if you are not comfortable with the price rate isn't it? That's why others take some ride when there is such big dump or even a minor one.
They will wait for such price where price decrease will be stopped or will be drastically be slowed. After that, they will consider that point if they want to purchase again some coins.