I'm afraid that I don't understand your requirements. From the image it seems that the thumb is in the center.
However, you can try to set HorizontalAlignment
of the thumb (ellipse) to Center. If this does not help, could you please send us a sample project with your implementation and customization and maybe images with actual and expected result so that we can help you further.
Thank you in advance for your cooperation.
Telerik by Progress
Telerik UI for WPF is ready for Visual Studio 2017 RC! Learn more.