First, note the and indicators indicate a solid down trajectory. Also note that the has been for some time now, since before the new year (2016) in fact. Note further the strong selling pressure via the OBV.
The optimal time to enter a short position? Wait for a short covering rally sometime in the next week following the massive dive from today, similar to what we saw in oil earlier this week. It will look head for the bound from above, but most likely turn sharply after a Heikin Ashi candle when the market ranges for a day or so.