The DropPosition is determined from the relative mouse position during the drag operation, which is sometimes not always entirely correct. This is one of the reasons why we started implementing a new drag and drop mechanism (I have already mentioned it in your other thread).
You can use its GetPosition method, in the DragEventArgs in i. e. the DragOver event, to get a correct relative mouse position and determine the exact drop position of the dragged item.
Hope this helps!
the Telerik team
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