The push-fit connectors used for connecting the Bowden tube to the extruder gives a bit play of the bowden tube, this decreases the effective amount of retraction. In order to counteract this there are two solutions.
Pushing the bowden tube hard and at the same time pull the plastic part of the connector upwards, it is quite fiddly but it works ok.
Using a small clip to insert below the collar of the plastic part, this takes away the play of the connector.
This would allow you to reduce your retract and make your retracts more consistent. This will also help if you have problems that the bowden tube gets pushed out of the connector. Also remember to
cut 5 mm off the bowden tube if you have pushed it out, the metal clips inside the connector needs a fresh surface in order to grip the tube correctly.
For normal use the gripping force should be more than enough, if you have temperature problems or foreign debris that clog your nozzle the bowden will get pushed out since the extruder does not slip. There could also be a build up on burn plastic inside the nozzle that inhibit correct heat transfer to the filament. For the E3D nozzles I have found that cleaning it out with a 2.0 mm drillbit by hand is a good way to remove any burnt residues.
You can download the clip for 1.75 & 3.0 mm here at http://www.bondtech.se/stl
Print with a layerheight of 0.20 mm, fintune your needed thickness by scaling the models in Z.