Hi Stefan,
I have found this article to be of help in my collimation.
http://www.astro-baby.com/collimatio...on%20guide.htm
Hope this helps.
Also note, some collimators themselves may not be perfectly collimated.
There are articles on IIS to identify and rectify this.
For example: If you hold the collimator steady (eg in eyepiece holder) and rotate it 360 degrees, does the red dot stay centred?