>It doesn't work.
>The bottoms of the texts still don't match.

I think Viv is correct, you want to align baselines not bottoms. This works for me:

<Grid>
  <FlowDocumentReader IsHitTestVisible="False">
    <FlowDocument>
      <Paragraph>Name:<Run FontSize="44">Itzhak Perlman</Run></Paragraph>
    </FlowDocument>
  </FlowDocumentReader>
</Grid>

Edit: That's probably not how your going to want to do it in your app. But that is a way to get WPF to do the alignment work for you.

This is probably the best way to do it in your app, assuming that the font sizes will always be the same:

<StackPanel Orientation="Horizontal">
  <Label Margin="0,33,0,0" Content="Name:"/>
  <Label Content="Itzhak Perlman" FontSize="44"/>
</StackPanel>