How do I change RichTextBox paragraph spacing?

I did it with style (pun indented)

<RichTextBox  Margin="0,51,0,0" Name="mainTextBox" >
        <RichTextBox.Resources>
            <Style TargetType="{x:Type Paragraph}">
                <Setter Property="Margin" Value="0"/>
            </Style>
        </RichTextBox.Resources>
    </RichTextBox>

Using Line Height

RichTextBox rtb = new RichTextBox();    
Paragraph p = rtb.Document.Blocks.FirstBlock as Paragraph;    
p.LineHeight = 10;