Scale while keep horizontal align

I have WPF control:

<UserControl x:Class="MyProject.LabelWithUnit"
             xmlns="http://ift.tt/o66D3f"
             xmlns:x="http://ift.tt/mPTqtT"
             xmlns:mc="http://ift.tt/pzd6Lm" 
             xmlns:d="http://ift.tt/pHvyf2" 
             mc:Ignorable="d" 
             d:DesignHeight="30" d:DesignWidth="150">
       <Viewbox StretchDirection="Both" Stretch="Uniform">
            <Grid Width="150">
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="*" />
                    <ColumnDefinition Width="Auto" />
                </Grid.ColumnDefinitions>
                <Label x:Name="ValueLabel" Grid.Column="0" Content="1013.0" Margin="0" VerticalAlignment="Center" HorizontalAlignment="Center" FontWeight="Bold" FontSize="18"/>
                <Label x:Name="UnitLabel" Grid.Column="1" Content="m/s" Margin="1" FontSize="10" />
            </Grid>
        </Viewbox>
</UserControl>

Now my control behave like this:

I want it to look like this instead, when i change width:

So, "m/s" part should always stick to top-right corner and numeric part should stay somehow near middle. When i increase Height of my control, it should scale both Labels.