Wpf – How to make a WPF TextBlock show the text on multiple lines

i have a wpf window where i have a stackpanel with two viewports in it – each Viewport with a textblock in it.

<Grid>
    <StackPanel VerticalAlignment="Center" Orientation="Vertical" >
        <Viewbox Margin="100,0,100,0">
            <TextBlock x:Name="headerText" Text="Lorem ipsum dolor" Foreground="Black"/>
        </Viewbox>
        <Viewbox Margin="150,0,150,0">
            <TextBlock x:Name="subHeaderText" Text="Lorem ipsum dolor, Lorem ipsum dolor, lorem isum dolor, Lorem ipsum dolor, Lorem ipsum dolor, lorem isum dolor, " TextWrapping="Wrap" Foreground="Gray" />
        </Viewbox>
    </StackPanel>
</Grid>

What i would like to achieve is that the top textblock is the heading with a bigger text. The second textblock is the sub heading with a smaller text. No matter how much text there is for either the heading or the subheading the font should dynamic become smaller/bigger. My problem is that i would like the subheading to be fixed width. This means that, the font should be a percentage (70%) of the heading and wrap to multiple lines, depending on how much text i have. I enclosed the code I have thus far… im missing something with that subheading, cant figure out what. Cheers

Edit
Basically what i want achieve is that the sub header wraps the text so it can expand it downwards with the font being a 70% of the heading – no matter how big that font is.