Wyckoff also talked a bit about 50% retracements - Wyckoff used...

Wyckoff also talked a bit about 50% retracements -

Wyckoff used the 50% retracement level as his line in the sand for pullbacks. A shallower pullback shows relatively weak selling pressure and underlying strength, while a deeper pullback reflects relatively strong selling pressure and underlying weakness. Once a stock hits the 50% sweet spot, Wyckoff would look for a high volume reversal surge to signal a continuation of the prior advance.

http://stockcharts.com/school/doku.php?id=chart_school:market_analysis:wyckoff2

Cheers