As the DocumentRuler and the RadRichTextBox are normally shown in the same grid row, the RadRichTextBox adjusting its position using the Margin property, you should also do the same when calculating the drop position. Please find attached a demo project with the following setup:
A RadTreeView with some items and a RadRichTextBox are placed on the main page. You can drag and drop items from the tree view in the editor. The editor is set to be in Paged layout mode
. In addition, the caret location is shown and the inserted text depends on the style of the current span.
If you are experiencing any issues get back to us with additional details on what to change in the demo.