Oh my, where to start..
well, first, thats a crazy flash crash that is rare..

second, if you did buy a dip off that broken support, you would be quite happy no matter where you bought because the price returned right back to $300 in a few minutes.. But if you had set a stop you would have gotten stopped out at a low price and would be crying in your soup.

Third, you never get too big in any position, so if the unthinkable ever happens you will still be ok.. only trade 10% of your account at any given time if your worried about getting struck by lightning.