WPF grid column 50% width

I need 3 columns in my Grid Layout, while looking like this:

1. 50% of the window
2. rest of the place
3. 30px wide

But I am unable to set this up, do any of you have idea ho to achieve this?

I tryed to toy with the *, but it always parse the width without the column with fixed width and it does not looks good.

So this does not achieve what I need:

               <Grid.ColumnDefinitions>
                <ColumnDefinition Width="*"/>
                <ColumnDefinition Width="*"/>
                <ColumnDefinition Width="30"/>
            </Grid.ColumnDefinitions>

Thank you for help, David.

I would create Grid with two columns and in the second column another Grid.

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="*"/>
        <ColumnDefinition Width="*"/>
    </Grid.ColumnDefinitions>
    <Grid Grid.Column="1">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="30"/>
        </Grid.ColumnDefinitions>
    </Grid>
</Grid>

Another option is to handle SizeChanged event of Window and calculate the width of the first column.

<Grid x:Name="MyGrid">
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="*"/>
        <ColumnDefinition Width="*"/>
        <ColumnDefinition Width="30"/>
    </Grid.ColumnDefinitions>
</Grid>

Code behind

private void Window_SizeChanged(object sender, SizeChangedEventArgs e)
{
    MyGrid.ColumnDefinitions[0].Width = new GridLength(e.NewSize.Width / 2);            
}