Hi Guillaume,
Thank you for contacting Syncfusion support.
Currently we can only set the position of FloatWindow by SetFloatingWindowRect() method of DockingManager. We have created a class named WindowPosition to return the center position of the Window by invoke the Method GetWindowCenterPoint(). We have prepared a sample that tries to meet your requirement. In this sample we have created the FloatWindow at center point by SetFloatingWindowRect() with center position of Window. We have attached the sample and you can download the sample from the following location.
Sample link: DockingManagerSample.zip
Please let us know if you need any further assistance,
Regards,
Thirumurugan Arumugam.