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The angle you can use will depend on the quality of steel in your knife, the geometry of the blade, the heat treatment etc

20 is fine for most, you can go lower with better quality knives/steels.

As superfly says, around 20 degrees will suit your wusthof for general work.

Try using the marker technique. Run it along the edge and sharpen until it is removed all the way along.

Kitchen knife forum at knifeforums http://www.knifeforums.com/forums/showforum.php?fid/118/

This may also be of interest, the keeping sharp forum...