If you just want to have your header font a little bit bigger then the rest, you can use ScaleTransform. so you do not depend on the real fontsize.
<TextBlock x:Name="headerText" Text="Lorem ipsum dolor">
<TextBlock.LayoutTransform>
<ScaleTransform ScaleX="1.1" ScaleY="1.1" />
</TextBlock.LayoutTransform>
</TextBlock>