Hi Thomas,
Thank you for contacting Syncfusion support.
We have checked your query "Setting the DP after DockingManager control has been loaded, doesn't work". We have prepared the sample to meet your requirement, in this sample we have set the SetFloatingWindowRect attached property of DockingManager to update the float window position and size. Please download the sample for the same from the below link.
Sample:
DockingManager_CodebehindRegards,
Marimuthu S.