I would vote for either a 38% retracement to 1521 with a 50% retracement to 1442 being a less likely "worst case" scenario. I think this is a correction in a cyclical bull market that still has another 12-14 months or so to fully play out, with the Nasdaq eventually reaching reaching between 2000-2300. No way do I see us retesting the October lows this year, however.
P.S. If I am correct with regarding to Nasdaq high during this cyclical bull, it will be the Mother-of-All shorting opportunities.