I am glad to hear that you have found a solution. I will post my suggestion here, so the community can benefit from it.
You can do this by accessing the paragraph that is currently pointed at the caret position. Here is a sample:
Paragraph paragraph = box.Document.CaretPosition.GetCurrentParagraphBox().AssociatedParagraph;
lineSpacing = paragraph.LineSpacing;
spacingAfter = paragraph.SpacingAfter;
Later you can modify these settings by using the appropriate methods of RadRichTextBox
If you have other questions, do not hesitate to write us.
the Telerik team