I'd say between here and $540 is a really good level to buy for a longer term hold.
Do you mean from $640 to $540?
I think the price of bitcoin might not drop below $600 in the short term.
No one have to wait that the price of bitcoin will drop to %540 as if we do not buy now and will wait then we will miss the train as the price of bitcoin is now increasing but not decreasing so we have to buy it now and have to store for longer time.