Thank's
I think the problem is as Paul has discribed it, since each line must have a dimension it can't be "exactly" on the spot.
For me that's OK, 'cause it does not affect my application in general. It just looks a bit strange to see a cross in a circle that should be in the middle and it isn't if your in the wrong scale (the cross has a size independant of scale, the circle goes with the scale). Try it with a cross that fills a small circle.
Anyway thanks, I continue on other problemspot of my app.